Let-Down Ratio Calculator: Formula, 2 Conventions and 6 Worked Examples

The let-down ratio is the share of masterbatch a plastics processor blends into natural resin, and it sets the additive level in the finished part: the level equals the masterbatch active content multiplied by the masterbatch share of the blend. The same dose is written two ways, as 5 % or as 19:1, so which number did the supplier mean? A 35 wt% carbon black masterbatch let down at 5 % puts 1.75 wt% carbon black into the part, and masterbatch active content itself typically runs 40 to 65 wt% (15 to 80 wt% in extreme grades), added at 1 to 5 % of the blend for most types.

This page runs 3 calculator modes, forward from a dose to a level, backward from a target to a dose, and sideways into batch weights, and states the single formula behind all three. It converts between the percentage and n:1 conventions, separates the percent-of-blend basis from the percent-on-resin basis, and reports every result in weight percent and parts per million alongside it. Six worked examples run the arithmetic on pressure pipe, an antioxidant masterbatch and the basis trap in numbers, and a closing section states what the arithmetic cannot decide and checks a result against a food-contact ceiling.

Every figure on this page is checked against primary sources; see our methodology and fact-checking process.

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Table T1. What the calculator returns.

Typical values are illustrations, not recommendations. The value for any grade comes from its supplier's technical data sheet.

How to Use the Let-Down Ratio Calculator#

The calculator works in 3 modes: forward from a masterbatch dose to the additive level in the part, backwards from a target level to the dose, and sideways from either into kilograms per batch. Each mode rearranges the same product rule, final additive level equals active content multiplied by masterbatch share, so switching modes never changes the underlying arithmetic, only which variable the calculator solves for. Only gravimetric loss-in-weight feeders reach ppm accuracy at the machine, which is the practical reason ppm-level additives such as clarifiers and slip agents are dosed as a masterbatch or a one-pack rather than metered directly as powder.

Mode 1: from a masterbatch dose to the additive level in the part#

Mode 1 answers the question a processor asks at the machine: if the bag says 35 wt% and the recipe says 5 %, how much of the additive is actually in the part?

Mode 1 takes three inputs:

• Masterbatch active content in wt%, read from the supplier technical data sheet
• The dose, entered as a percentage of the blend or as a ratio n:1, on one toggled field
• Batch size in kg, optional

Mode 1 returns three outputs:

• Additive level in the part, in wt% and ppm
• The same dose written in the other convention
• Masterbatch and natural resin weight in kg

A 35 wt% carbon black masterbatch let down at 5 %, which is 19:1, gives 1.75 wt% carbon black in the part (Ampacet). The active content field starts empty with a placeholder rather than a default of 50 %, because active content is a supplier value and is never a reasonable default.

Mode 2: from a target additive level to the masterbatch dose#

Mode 2 runs the same product rule backwards: it divides the target level by the masterbatch active content, which is how a formulator turns a specification into a dosing rate.

masterbatch share of the blend (%) = target level (wt%) / active content (wt%) x 100

Mode 2 takes three inputs:

• Target additive level in the part, in wt% or ppm, on one toggled field
• Masterbatch active content in wt%
• Batch size in kg, optional

Mode 2 returns three outputs:

• Required dose as a percentage of the blend, and as a ratio
• Masterbatch weight in kg per batch
• A non-blocking warning when the required dose falls outside the usual 1-5 % window

A compounder targeting the ISO 4427 carbon black band of 2.0 to 2.5 wt% for PE pressure pipe uses Mode 2 to find the dose directly, instead of guessing a starting percentage and checking the result afterwards. That target itself rarely comes from this calculator: it comes from a standard, a customer specification or the technical data sheet for the additive family in question, and every family of plastic additives is indexed and linked from plastic additives, from plasticizers and flame retardants to the antioxidants and fillers used in the worked examples below.

Mode 3: batch weights in kilograms#

Mode 3 turns a percentage into the two weights a hopper actually needs: the masterbatch share of the batch and the natural resin that makes up the rest. It takes the masterbatch share, from Mode 1 or Mode 2, together with the batch size in kg, and applies the formula below.

masterbatch (kg) = share (%) / 100 x batch weight (kg) natural resin (kg) = batch weight (kg) - masterbatch (kg)

At a 5 % share, a 1,000 kg batch needs 50 kg of masterbatch and 950 kg of natural resin, which lines up with the commonly quoted figure of 25 kg of masterbatch per tonne at a 2.5 % share. The calculator's batch field always means the finished blend, masterbatch plus resin together, not the resin alone; a dose given "per tonne of resin" is a different basis, covered in the basis section below.

Presets: masterbatch types and the let-down values behind them#

The calculator ships with 6 presets, and every one of them carries the source it comes from rather than a round number. Selecting a preset fills the active content and dose fields and shows the source line beneath them, so no figure floats free of its origin. Composition, active content and the full let-down table by masterbatch type sit on masterbatch, the hub page this calculator draws its presets from.

Table T2. Calculator presets.

Ratios in this table are calculated here from the percentage using 1/(n+1). "See supplier" means our source library holds no active-content value for that type; enter the value from the technical data sheet.

Three further masterbatch let-down figures circulate in industry sources: antiblock masterbatch near 1-3 %, processing-aid masterbatch near 2-5 % and anti-scratch siloxane masterbatch near 50 % active. None of the three is confirmed closely enough by a primary source to become a preset, so none of them appears in this table or elsewhere on this page.

The Let-Down Ratio Formula#

The let-down ratio formula is a single product: the additive level in the finished part equals the masterbatch active content multiplied by the masterbatch share of the blend.

Final additive level (wt%) = active content (wt%) x masterbatch share of the blend (%) / 100

A 35 wt% carbon black masterbatch let down at 5 % gives 1.75 wt% carbon black in the part (Ampacet). This single product rule is the same equation Mode 1, Mode 2 and Mode 3 each rearrange to solve for a different unknown, and it is exact once the active content and the dose are both known. Nothing about carrier compatibility, dispersion or feeder accuracy enters into it, which is the subject of a later section.

Converting between the two let-down ratio conventions#

The same dose is written 2 ways: as a percentage of the blend, such as 5 %, or as a resin-to-masterbatch ratio, such as 19:1, and Ampacet's pipe guide uses both in a single sentence. Ampacet writes "5 % (19:1)" and "6.5 % (14:1)" together, and the same source writes coloured pressure pipe as "2-4 % (49:1 to 24:1)", so both conventions are in daily use side by side. A ratio of n:1 means 1 part masterbatch to n parts natural resin, so the masterbatch share of the blend is 1/(n+1): 19:1 gives 1/20, which is 5 %; 49:1 gives 1/50, which is 2 %; 24:1 gives 1/25, which is 4 %.

masterbatch share (%) = 100 / (n + 1) n = 100 / masterbatch share (%) - 1

Table T3. Ratio to percentage.

Percentages and kilogram figures are calculated here from the ratios using 1/(n+1). The 6.5 % in the last row is the value Ampacet publishes; 6.67 % is the exact arithmetic.

A 9:1 mixing ratio, in these plastics terms, means 9 parts natural resin to 1 part masterbatch, which is 1/10 of the blend, or 10 %. The same table shows a rounding the industry writes past every day: 14:1 is exactly 1/15, which is 6.67 %, and Ampacet publishes that dose as 6.5 %, not 6.67 %. This page states the rounding openly rather than silently correcting the supplier's own figure.

A second reading of n:1 also circulates: some trade sources treat 20:1 as loosely equal to 5 %, which is the 1/n reading rather than the exact 1/(n+1) reading. This is not an error so much as a second convention, and a data sheet's own worked pair usually shows which one it follows. This calculator uses the exact 1/(n+1) reading throughout, the same reading behind every Ampacet pair above, and labels every ratio output "ratio (resin : masterbatch)" so the convention is never ambiguous.

Percent of the blend or percent on the resin?#

A dose written as a percentage can mean two different things: the masterbatch as a share of the finished blend, or the masterbatch added on top of a fixed weight of resin. 25 kg of masterbatch into 1,000 kg of resin is 2.5 % on the resin, and, calculated here, 2.44 % of the 1,025 kg blend that results. 5 kg into 100 kg of resin is 5.0 % on the resin, and, calculated here, 4.76 % of the 105 kg blend.

The on-resin basis is the same idea used elsewhere in formulation: the polymer is always counted as 100 parts and everything else is added on top of it, which is the logic behind PHR (parts per hundred resin), the fourth dosage unit alongside weight percent and ppm.

The gap between the two bases is small at an ordinary 1-5 % let-down and large at filler-masterbatch loadings, where the masterbatch itself can be a third of the finished weight. This calculator reports the blend basis as its primary result and shows the on-resin equivalent beside it, so a figure copied from a data sheet written on the other basis is never silently misread.

From weight percent to ppm#

Stabilizers, clarifiers and slip agents are specified in parts per million rather than in percent, so the calculator reports both: 1,000 ppm is 0.1 wt%. A part per million is a mass fraction multiplied by 10^6.

ppm = wt% x 10,000

Ampacet's antioxidant masterbatch example uses this conversion directly: a 2.5 % addition of that masterbatch gives 1,000 ppm of active antioxidant in the finished part, the shortest worked pair in the whole let-down system.

Six Worked Let-Down Ratio Examples#

The 6 examples below run the formula in both directions on a pressure pipe, an antioxidant masterbatch, the basis trap in numbers and a food-contact ceiling check. Each one states its requirement and source first, then its masterbatch and source, then the arithmetic, labelled calculated here wherever this page performs it rather than a supplier.

1. Black masterbatch for an HDPE pressure pipe#

A PE100 pressure pipe has to contain 2 to 2.5 wt% carbon black under ISO 4427, and the standard 35 wt% black masterbatch reaches that band at a let-down of 5 to 6.5 %, which is 19:1 to 14:1 (Ampacet).

1. Requirement. ISO 4427, with EN 12201-1 in Europe, sets carbon black at 2.0-2.5 wt% in PE pressure pipe.
2. Masterbatch. The North American standard black pipe masterbatch is 35 wt% carbon black, carried in MLDPE or LLDPE below 20 melt index; masterbatches above 40 wt% carbon black are avoided, and grades, particle size and the UV function of carbon black in plastics decide which one is used.
3. Arithmetic. 35 wt% x 5 % / 100 = 1.75 wt%; 35 wt% x 6.5 % / 100 = 2.275 wt%, rounded to 2.3 wt% (calculated here).
4. Check. At the bottom of the published range the result, 1.75 wt%, sits below the standard's 2.0 wt% floor, so a published let-down range is a starting point for trials, not a specification on its own.

2. The same pipe, calculated backwards from the target#

Run Mode 2 on the same pipe and the answer is a single number: a 2.0 wt% carbon black target from a 35 wt% masterbatch needs a 5.7 % let-down, which the calculator also writes as 16.5:1.

1. Target. 2.0 wt% carbon black, the lower edge of the ISO 4427 band.
2. Masterbatch. The same 35 wt% carbon black masterbatch as Example 1.
3. Arithmetic. 2.0 / 35 x 100 = 5.7 % (calculated here), which converts to 100 / 5.7 - 1 = 16.5, so 16.5:1 (calculated here). Both figures are arithmetic on the two source values above, not published numbers.

The rest of the pipe package, including the antioxidant level the standard tests by OIT, is covered on additives for polyethylene, alongside the carbon black requirement handled here.

3. Coloured pressure pipe: reading 2 to 4 % as 49:1 to 24:1#

Coloured pressure pipe runs at 2 to 4 % masterbatch, which Ampacet writes as 49:1 to 24:1, and those two pairs are the cleanest proof of which ratio convention the industry means. 2 % is exactly 1/50, and 4 % is exactly 1/25, with no rounding at either end, unlike the black masterbatch pair in Example 1. Non-pressure pipe often runs at a lower let-down still, per the same source. Colour masterbatch selection and the difference between pressure and non-pressure pipe practice are compared on additives for plastic pipes, a sibling reference to this calculator.

4. Antioxidant masterbatch: 2.5 % for 1,000 ppm#

Ampacet's antioxidant guide gives the shortest possible worked example: 2.5 % of the masterbatch puts 1,000 ppm of active antioxidant into the polymer.

1. Requirement. 1,000 ppm active antioxidant in the part, which is 0.1 wt%.
2. Masterbatch. A 2.5 % addition of the antioxidant masterbatch reaches that level (Ampacet).
3. Arithmetic. Working the product rule backwards, 0.1 / 2.5 x 100 = 4 wt%, so the masterbatch implies a 4 wt% active content, and 2.5 % converts to 39:1 (both calculated here). Ampacet does not publish the masterbatch's active content directly, so 4 wt% is an implied figure, never the supplier's own stated number. Which grade covers that 1,000 ppm, primary or secondary, and how the two are paired, is covered on antioxidants for plastics.

5. 25 kilograms per tonne: the basis trap in numbers#

The most quoted let-down example in the literature, 25 kilograms of masterbatch per tonne of base polymer, is published as 2.5 %, and that figure is on the resin, not on the blend.

1. Source figure. 25 kg of masterbatch per 1,000 kg of base polymer, published as 2.5 % (Wikipedia, Masterbatch).
2. On the resin. 25 / 1,000 x 100 = 2.5 % (calculated here), matching the published figure exactly.
3. Of the blend. 25 / 1,025 x 100 = 2.44 %, which is 40:1 (calculated here), because the 1,025 kg blend includes the masterbatch itself in the denominator.

A gap of 0.06 percentage points between 2.5 % and 2.44 % is irrelevant for a colour dose and directly relevant for a regulated additive sitting close to its ceiling, the subject of the next example.

6. Checking the result against a food-contact ceiling#

A calculated level is only useful once it is compared with the limit that applies to the part, and for carbon black that limit is 2.5 % by weight of the polymer in both the EU and the US.

1. EU limit. Carbon black sits on the Union list of EU 10/2011 as FCM 411, authorised at a maximum of 2.5 % w/w in the polymer.

2. US limit. The matching US clearance is set out on FDA food contact rules, 21 CFR 178.3297, which caps high-purity furnace black at 2.5 % by weight of the polymer.

3. Result. The upper end of Example 1, 2.3 wt% at a 6.5 % let-down, clears the 2.5 % ceiling with about 0.2 percentage points to spare.

4. Denominator check. The ceiling is written on the polymer, and the masterbatch carrier is itself polymer, so the carrier resin entering the matrix has to be added to the base resin before the ceiling is checked.

carrier entering the matrix (wt% of blend) = share (%) x (100 - active content (%)) / 100

At 6.5 %, a 35 wt% active masterbatch adds 6.5 x (100 - 35) / 100 = 4.2 wt% carrier resin to the matrix (calculated here), so the correct denominator is base resin plus carrier, not base resin alone. This calculator flags a ceiling only for the substances whose limits are in its source library, it is not a compliance tool, and no result here counts as "FDA approved" clearance.

What the Let-Down Ratio Calculator Cannot Decide#

The arithmetic is exact, but 5 things the calculator cannot see decide whether the calculated dose actually works in production.

• Carrier compatibility with the base resin. Black pipe masterbatch uses an MLDPE or LLDPE carrier below 20 melt index; a mismatched carrier can affect the finished part even when the additive level is correct.
• Dispersion quality at the chosen dose. Pipe compounders rate dispersion per lot on a 1-5 microdispersion scale, and the same wt% figure can come from a well-dispersed batch or a poorly dispersed one.
• Feeder accuracy at the machine. Gravimetric loss-in-weight feeders are needed to reach ppm-level accuracy, which is the practical reason ppm additives are dosed as masterbatch or a one-pack rather than metered as raw powder.
• Strength differences between grades at the same nominal active content. Two masterbatches both labelled 35 wt% can still perform differently at the same let-down.
• Delivery form. Powder, dust-free granules, pastilles and liquid concentrates dose differently on the same equipment; the routes are compared on additive product forms.

Cost in use should be checked separately from active content: it equals the masterbatch price multiplied by the let-down fraction, so a stronger masterbatch at a lower dose can cost less per part even at a higher price per kilogram. That comparison runs on the additive dosage and cost-in-use calculator, a sibling tool to this one, since no masterbatch price sits in our sources for this page. How carrier and base resin actually mix during compounding is covered on plastic compounding, the process this calculator's output feeds into.

Five Mistakes When Setting a Let-Down Ratio#

Five mistakes turn a correct let-down calculation into a wrong part, and four of them are unit errors rather than chemistry errors.

1. Reading a ratio with the wrong convention. Taking 20:1 as 5 % when the supplier means the exact 1/(n+1) reading, 1/21 = 4.76 %, understates the dose; the reverse mistake, reading an exact ratio loosely, overstates it.
2. Mixing the bases. Adding 5 kg of masterbatch to 100 kg of resin and calling the result 5 % of the blend is wrong: the blend weighs 105 kg, so the correct figure is 4.76 %.
3. Assuming the active content. Masterbatch active content runs from 15 to 80 wt%, so copying a dose from one grade onto another grade without checking its data sheet changes the level in the part; the level each additive family needs is tabulated on additive dosage levels in plastics.
4. Checking a regulatory limit on the wrong denominator. A limit written as % w/w of the polymer counts the masterbatch carrier resin as polymer, not only the base resin, as Example 6 above shows.
5. Dosing a ppm-level additive by percentage on a volumetric feeder. Reaching ppm accuracy needs a gravimetric loss-in-weight feeder, which is exactly why those additives ship as masterbatch in the first place rather than as loose powder.

Where the Numbers in This Calculator Come From#

Every preset in this calculator carries a source, and the sources fall into 4 classes that this page keeps visibly apart.

• Primary standards. ISO 4427 and EN 12201-1 set the carbon black band for PE pressure pipe; EU 10/2011 and 21 CFR 178.3297 set the food-contact ceiling.
• Supplier technical documents. Ampacet's pipe compounding guide supplies the black and coloured pipe let-down pairs and the antioxidant masterbatch example; Plastiblends supplies the antifog masterbatch let-down.
• Encyclopedic definitions. The generic 40-65 wt% active-content band, the usual 1-5 % addition and the 25 kg per tonne example come from the Masterbatch entry on Wikipedia.
• Arithmetic done here. Every ratio, kilogram figure and implied active content this page calculates from those sourced values is labelled calculated here, and is never presented as a supplier's own number.

This page spells the term masterbatch as one word throughout, the spelling used by the supplier documents and standards above; some older trade literature still writes it as master batch, two words, with the same meaning. The rest of the formulation workflow, from selecting an additive to compounding it into the batch, is indexed on plastic formulation and additive processing.

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Let-Down Ratio and the Other Ratios It Gets Confused With#

In plastics compounding a let-down ratio always means the share of masterbatch in a resin blend, and the same phrase, and the same arithmetic, are used for very different mixtures elsewhere. Eleven of the top results for a bare "let down ratio calculator" search return tools for paint, epoxy, cleaning dilution or cosmetics rather than for plastics, which is the reason this page states its border explicitly before returning to the plastics content above.

Dilution, mix and let-down ratios in paint, ink and casting resin#

Paint reducers, cleaning concentrates, flexographic inks and two-part casting resins are all dosed with n:1 mix ratios, and the ratio-to-percentage conversion above works for them too. The arithmetic, share equals 1/(n+1), is the same regardless of what is being diluted. The values, the governing standards and the health and safety limits are not the same, and coatings, inks, adhesives and cosmetics fall outside the scope of this plastics reference.

LDR is not L/D: screw geometry uses the same two letters#

LDR and L/D are two different ratios: the let-down ratio is a dosing number, while L/D is the length-to-diameter ratio of the screw that melts the blend. A vented two-stage compounding screw typically runs an L/D of 36:1, a geometry figure with no arithmetic connection to any masterbatch dose; the two appear together on search results only because both are written as a ratio in plastics processing.