PHR (Parts per Hundred Resin): 6 Worked Conversions to wt%, ppm and Let-Down Ratio

PHR means parts per hundred resin: the polymer in a plastic formulation is fixed at 100 parts by weight, and every additive is written as its weight relative to those 100 parts. That fixed basis is what makes phr useful and what makes it easy to misread, because 10 phr is never 10 wt%. This page performs 3 core conversions in heading order, phr to wt%, wt% back to phr, and phr to a batch weight in kilograms, then places the two neighbouring dosage units, ppm and let-down ratio, on the same fixed-basis logic.

Six worked examples carry the arithmetic: a rigid PVC pressure pipe recipe published by the Plastics Pipe Institute at 108.03 phr, a halogen-free flame retardant cable compound at 160 phr, a PVC cable insulation recipe converted to weight percent, a flexible PVC reverse conversion, a 500 kg batch weight and a 35% carbon black masterbatch. The page also states which unit each polymer and additive family normally uses, which basis a regulatory limit is written on, 5 mistakes that turn a correct recipe into a wrong one, and links to the two calculators that automate every conversion below.

phr and wt% describe the same recipe; they never give the same number unless the compound is pure polymer.

What Does PHR Mean in a Plastic Formulation?#

PHR (parts per hundred resin) is a weight-based dosage unit in which the polymer is set to 100 parts and every other ingredient is expressed as parts by weight against that fixed 100. So why not simply write every recipe in percent? Because a percentage recalculates every time an ingredient is added or removed, while a phr value stays fixed as the recipe changes, which is the property the next section explains in full. Flexible PVC illustrates how far phr and wt% can drift apart in exactly the compound type where phr is used most: a plasticizer level of 5 to 65 wt% of the compound is 5 to 186 phr of the resin, a gap that grows because the plasticizer is often the largest single ingredient after the polymer itself. Dosage across a formulation runs in 4 bands, from parts per million for trace stabilizers, through 0.05 to 1 wt% for the majority of stabilizer packages, to 1 to 10 phr for PVC stabilizers, lubricants and impact modifiers, up to 10 to 70% for plasticizers, fillers and mineral flame retardants.

Every phr value in a recipe belongs to one of the 43 families of plastic additives, and each family has its own usual unit, set out family by family later on this page. A phr number is not a percentage of anything visible in the finished part; it is a ratio against a basis, the polymer's 100 parts, that never appears as a line item in the compound itself.

Why the resin is always 100 parts#

Fixing the polymer at 100 parts means an ingredient keeps its number when the recipe changes: adding 5 phr of filler leaves the stabilizer at 0.70 phr, while in weight percent every other value would have to be recalculated. A compounder can therefore add or remove one ingredient without touching the phr value written for every other ingredient in the recipe, because each value is measured against the fixed 100 parts of polymer, not against the shifting total of the whole compound. That convenience carries a cost: the parts in a real recipe add up to more than 100, reaching 108.03 parts in the Plastics Pipe Institute's pressure-pipe recipe and 260 parts in a highly filled cable compound, so a phr number is always larger than the weight percent it corresponds to once the total is counted.

Parts per hundred resin or parts per hundred rubber?#

Both expansions are correct: plastics compounders read PHR as parts per hundred resin and rubber compounders read it as parts per hundred rubber, and the arithmetic is identical because both fix the base polymer at 100 parts. The abbreviation also appears as "pphr" and, informally, as "parts," and polyurethane foam uses a related unit, pphp (parts per hundred polyol), covered in full below the contextual border of this page. Outside materials engineering the bare acronym is ambiguous, so this reference and most technical documents write "parts per hundred resin" out in full on first use.

How Do You Convert PHR to Weight Percent?#

Convert phr to weight percent in 3 steps: add every phr value in the recipe including the 100 parts of polymer, divide each ingredient's phr by that total, then multiply by 100. The 3-step procedure below applies to any recipe already expressed in phr, whether it carries 3 ingredients or 12.

1. Total every phr value in the recipe, starting with the polymer's 100 parts.
2. Divide the ingredient's phr by that total.
3. Multiply the result by 100 to get the ingredient's wt% of the compound.

The phr to wt% formula#

The phr to weight percent formula divides one ingredient's parts by the sum of all parts in the recipe: wt% = phr_i / total phr x 100.

wt% of ingredient i = (phr of ingredient i / total phr of the recipe) x 100

The converted values must add up to 100.00; the phr values must not, because the polymer's 100 is a basis and not a share of the total.

Worked example 1: rigid PVC pressure pipe (108.03 phr)#

The Plastics Pipe Institute publishes a range composition for US PVC pressure pipe in TR-2 (2023), and its worked recipe totals 108.03 parts per hundred resin. The recipe covers PVC 1120 in cell class 12454 at a hydrostatic design basis of 4,000 psi, and every ingredient carries both a published range and a published example value.

Range composition and worked example from PPI TR-2-2023, Appendix C (PVC 1120, cell class 12454, HDB 4,000 psi). The wt% column is published in the same source.

The size of the gap between the two units is set by that single total: the tin stabilizer is 0.70 phr but 0.65 wt%, a difference of about 7%, and the same 7% gap applies to every other ingredient in the recipe because one shared total, 108.03, divides all of them. The full package and why every PVC compound carries a heat stabilizer are on additives for PVC.

Worked example 2 and 3: highly filled cable compounds#

At high loadings the two units drift far apart: a halogen-free flame retardant cable compound with 160 phr of aluminium trihydroxide contains 61.5 wt% of it, because the recipe totals 260 parts. An EVA/LLDPE-based halogen-free flame retardant (HFFR) cable compound carries aluminium trihydroxide (ATH) or magnesium dihydroxide (MDH) at 160 to 180 phr, and at the lower end of that range the flame retardant alone makes up more than three-fifths of the compound's weight. The full LSZH, PVC and XLPE packages are on additives for wire and cable compounds.

A second recipe, a PVC K70 cable insulation compound, shows the same conversion applied to a full ingredient list rather than a single dominant filler.

Formulation from a Huber reference recipe recorded in our source library (LOI 26-27%, UL 94 V-0 at 3 mm); ATH is given there as 45-100 phr and this example uses 45 phr. The wt% column is calculated from those phr values with the formula above and is not quoted from the source.

How Do You Convert Weight Percent Back to PHR?#

To go back from weight percent to phr, divide the ingredient's wt% by the polymer's wt% and multiply by 100, which is why a flexible PVC at 30 wt% plasticizer is a 42.9 phr recipe.

phr of ingredient i = (wt% of ingredient i / wt% of the polymer) x 100

A flexible PVC compound at 30 wt% plasticizer against 70 wt% PVC gives 42.9 phr (30 / 70 x 100). At the upper end of the flexible PVC range, 65 wt% plasticizer against 35 wt% PVC gives 186 phr (65 / 35 x 100), the same recipe expressed at the top of our source library's 5 to 65 wt% plasticizer band. The reverse calculation needs the polymer's own wt%, not the total of 100, and the polymer's wt% is not simply 100 minus the additive being converted unless the compound has exactly two ingredients. This is the single most common error in the public Q&A threads that answer this question: taking "100 minus the additive" as the polymer share works only for a 2-ingredient compound, and fails as soon as a third ingredient, such as a stabilizer or a filler, is added.

How Do You Convert PHR to a Batch Weight in kg?#

Multiply the batch size by each ingredient's share of the total phr to get its weight in kilograms.

weight of ingredient i = (phr of ingredient i / total phr) x batch weight

1. Total the phr values in the recipe.
2. Divide each ingredient's phr by that total.
3. Multiply the result by the batch weight in kilograms.

Scaling the PPI TR-2 pipe recipe to a 500 kg dry blend, calculated from the phr values in Table above, gives:

• PVC resin: 462.8 kg
• Calcium carbonate: 23.1 kg
• Paraffin wax: 5.6 kg
• Tin heat stabilizer: 3.24 kg
• Calcium stearate: 2.08 kg
• Titanium dioxide: 2.31 kg
• PE wax: 0.69 kg
• Pigment: 0.14 kg

Weighing accuracy sets a practical floor on how small an ingredient can be dosed neat: a 0.03 phr pigment is only 0.14 kg in a 500 kg batch, which is why the smallest ingredients in a formulation are usually dosed as a masterbatch or a one-pack rather than weighed out on their own. Gravimetric loss-in-weight feeders are needed to hold accuracy at that scale, and below roughly 1,000 ppm most plants dose the additive pre-diluted rather than neat.

PHR, wt%, ppm and Let-Down Ratio: How the Four Units Relate#

The four dosage units in plastics formulation differ only in what the number is measured against: 100 parts of polymer (phr), the whole compound (wt% and ppm) or the blend that goes into the machine (let-down ratio). Once the basis is identified, every one of the four converts into any other using the same two formulas already given above.

When ppm replaces wt%#

One thousand ppm is 0.1 wt%, and formulators switch to ppm below roughly 0.1% because writing a clarifier as 0.0175 wt% invites a decimal error. Parts per million is a mass fraction multiplied by 10^6, so it expresses the same trace quantity with whole numbers instead of a string of leading zeros. Clarifiers such as Irgaclear XT 386 run at 150 to 200 ppm, acid scavengers reach up to 1,000 ppm, and slip agents sit between 500 and 1,200 ppm, all bands where a ppm figure reads more cleanly than its wt% equivalent. Restabilizing a recycled polypropylene at 500 ppm of a primary antioxidant plus 1,000 ppm of a secondary antioxidant is a combined 0.15 wt% package; the grades used for that package are compared on antioxidants for plastics.

How let-down ratio converts into phr and wt%#

Final additive level equals the masterbatch's active content multiplied by the let-down.

final wt% of additive = active content of the masterbatch (wt%) x let-down (%) / 100

Masterbatch active content typically runs 40 to 65 wt% (with extremes from 15 to 80 wt%), and let-down at the machine usually runs 1 to 5% of the base polymer; 25 kg of masterbatch per tonne of polymer is 2.5% let-down. Composition and let-down ratios by type are on masterbatch.

A 35% carbon black masterbatch dosed at 5% let-down (written as both "5%" and "19:1") gives 1.75 wt% carbon black in the finished pipe wall; at 6.5% let-down (14:1) the same masterbatch gives 2.3 wt% carbon black, and masterbatches above 40% carbon black are generally avoided because they disperse less reliably at low let-down. To reach 2.0 wt% carbon black from a 35% masterbatch the let-down works out to 5.7%.

Both "5%" and "19:1" describe the same dose, and "1:50" and "50:1" are both written by different suppliers for the same dilution, so a formulation record always states which convention is meant. On a phr basis, a 5% let-down is 5.26 phr, because the let-down is a share of the whole blend and phr is a share of the polymer alone, so the two numbers are never identical even at the same dose. Carbon black grades and pipe practice for the worked example above are on black masterbatch.

Which Unit Does Each Polymer and Additive Family Use?#

PVC, rubber and peroxide-crosslinked compounds are written in phr, filled and reinforced thermoplastics in wt%, and trace stabilizers in ppm, because each unit keeps the numbers of its own recipe readable. Each polymer's usual package and its usual unit are set out under additives by polymer, while the table below gives one worked example per unit.

Conversions in the right-hand column are calculated with the formulas above and assume the compound contains only the ingredients named; they are illustrative, not recipe values.

Rigid PVC pipe carries its lubricant and filler package on the same phr basis as the table's first row, and PVC dry blending is where that package is mixed into the dry powder before extrusion. Table T4 above gives one example per unit rather than a per-family dosage catalogue; the level each family actually needs across all its applications is tabulated on additive dosage levels in plastics.

Which Basis Does a Regulatory Limit Use?#

Additive limits are written on 4 different bases: a share of the polymer, a share of the finished compound or article, parts per hundred resin, and milligrams per kilogram of food, and a recipe only complies once it has been converted onto the basis the rule uses.

Bases are quoted as the instruments write them. Consolidated EU text of 14 July 2026; eCFR current to 18 September 2026.

The carbon black case makes the point no other reference on this topic makes explicitly: 2.5% w/w in the polymer, the limit set by EU 10/2011 FCM 411, is not the same as 2.5% of a filled compound. In a compound that is 60 wt% polymer, the same rule allows 1.5 wt% of the whole compound, a lower absolute figure than the headline 2.5% suggests. Every instrument that sets a level for an additive is summarised in plastic additive regulations; no product on this page is described as "FDA approved", because the instrument, the section and the basis, not a blanket approval, are what the rule actually grants. An SML or an OML is never converted into a recipe level; both limit what migrates into food, not what the formulator may put into the compound.

Five Mistakes When Converting PHR in a Formulation#

Five errors turn a correct recipe into a wrong one: leaving the polymer out of the total, reading phr as a percentage, mixing bases, inverting a let-down ratio, and comparing cost by weight when the part is sold by volume.

1. Forgetting the polymer's 100 parts in the total. Dividing by the sum of the additives alone, instead of the additives plus the polymer's 100, inflates every resulting wt% value; the total should always start at 100.
2. Treating phr as a percentage. 0.70 phr is 0.65 wt% in the PPI TR-2 pipe recipe, and the gap only grows with loading, reaching 160 phr equal to 61.5 wt% in an HFFR cable compound.
3. Mixing bases in one sentence. "2.5%" means a share of the polymer under EU 10/2011 FCM 411 and a share of the whole compound almost everywhere else on this page; the basis should be named every time a percentage is quoted.
4. Reading a let-down ratio the wrong way round. 5% and 19:1 are the same dose, 1:50 and 50:1 are both written for the same dilution by different suppliers, and the additive's final level is the masterbatch's active content multiplied by the let-down, not the let-down figure on its own.
5. Comparing cost per kilogram instead of cost per part. Cost per kg of compound is the sum of each ingredient's weight fraction multiplied by its price, but cost per litre is that same figure multiplied by the compound's density, so a filler that lowers cost per kg also raises density and can fail to lower cost per part. Polymer densities set the starting point for that comparison: polypropylene runs 0.895 to 0.93 g/cm3, low-density polyethylene about 0.924 g/cm3, and high-density polyethylene about 0.961 g/cm3.

The rest of the workflow, from selection to compounding, is indexed on plastic formulation and additive processing.

Calculate It: the PHR and Let-Down Ratio Calculators#

Both conversions on this page are automated, so a full recipe can be checked in one pass instead of ingredient by ingredient. Entering a recipe once removes the risk of the 5 mistakes above, because the tool applies the fixed-basis formula to every ingredient at the same time.

Paste a whole recipe into the PHR to weight percent calculator and it returns both columns, phr and wt%, side by side for every ingredient. Cost per kilogram and cost per litre, the comparison behind mistake 5, are handled separately by the additive dosage and cost-in-use calculator, which converts a recipe into a cost-per-part figure once polymer and filler densities are entered.

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What Else Does PHR Mean Outside Plastic Compounding?#

The same three letters carry four other meanings, and only one of them is a plastics formulation unit. How a recipe in phr becomes pellets is on plastic compounding, the process step that follows every conversion on this page.

• Parts per hundred rubber, the elastomer-industry reading of the same acronym
• Parts per hundred polyol (pphp), the equivalent unit used in polyurethane foam
• The resin-to-hardener mix ratio used by casting-resin and epoxy suppliers
• A personal health record or a human-resources certification, unrelated to chemistry

phr in rubber compounding#

Rubber compounders write phr as parts per hundred rubber and fix the elastomer at 100 parts, so every formula on this page applies unchanged. The dosage levels used for accelerators and vulcanizing agents fall outside this reference's plastics-only border, so no rubber recipe or rubber-specific dosage is given here.

pphp in polyurethane foam#

Polyurethane foam uses pphp, parts per hundred polyol, which fixes the polyol at 100 parts in exactly the way phr fixes the resin. A reference flexible foam formulation runs at index 100 with 6 pphp of water as the blowing agent; the isocyanates, polyols and amine or tin catalysts that complete that recipe are outside the scope of this reference and are not covered here.

phr in casting resin, epoxy and coatings#

Casting-resin and adhesive suppliers use PHR for the weight of hardener per 100 parts of resin, which is a mix ratio rather than an additive level, and this reference does not cover it. Questions such as "what is a 2:1 ratio in resin" or "how do you calculate a 3:1 ratio resin" belong to that mix-ratio sense of the acronym, a different question from the additive-dosage question this page answers, and this reference does not answer them.

Non-chemistry meanings of PHR#

Outside materials work the acronym means a personal health record or a human-resources certification, which is why technical documents write it out as parts per hundred resin on first use. A bare search for "phr" also returns a Finnish business registry, none of which describes a plastics formulation unit.

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