How do you calculate contamination rate?

Divide the out-throw and prohibitive weight by the total sampled weight, then multiply by 100. With 8 kg of out-throw in a 250 kg sample, the contamination rate is 3.2%.

What is a good contamination rate for a recycled bale?

It depends on grade, but many fiber and PET specifications target 5% or lower. The 3.2% in this example sits comfortably 1.8 points under a 5% target, which is a sellable, in-spec result.

What counts as out-throw versus prohibitive material?

Out-throws are recyclables of the wrong type that lower grade; prohibitives are items that can damage equipment or render the bale unusable, like trash or hazardous material. Both are summed in the contamination weight here.

What does the gap to specification tell me?

It is your contamination rate minus the target. Here 3.2% minus a 5% target gives a gap of 1.8 points to the good side, meaning you have headroom. A positive number toward the target means you are in spec; exceeding the target means rework.

How big should my contamination sample be?

Large enough to be representative, typically a full grab or cut from the bale of tens of kilograms. The 250 kg total here is a robust sample; very small samples swing the percentage wildly on a single heavy item.

Contamination rate vs purity, what's the difference?

They are complements. If contamination is 3.2%, purity is 96.8%. Buyers often specify a minimum purity or a maximum contamination; this tool reports the contamination side and the gap to your stated limit.

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Bale Contamination Rate Calculator

Contamination rate is the percentage of out-throw and prohibitive material in a recovered bale or stream, and it is the single number that decides whether a load gets accepted, discounted or rejected at the mill. MRF quality managers, balers and commodity sales teams sample bales against ISRI or mill specifications and track this rate to protect price and avoid costly rejections. It matters because even a few points of contamination can drop a load from a premium grade to a salvage price, or trigger a chargeback. Knowing your rate and the gap to target lets you tune sorters and presort before the load ever leaves the dock.

What this calculator does

Estimate the contamination rate in an outbound bale or sampled stream and see how far it sits from the broker or specification target.
Use it when running an ISRI-style bale audit or composition study and you need a clean contamination percentage plus the gap to the spec you ship against.
It computes the contamination percentage from out-throw weight over total sampled weight, and the point gap to your specification target.

Formula used

Contamination rate = out-throw weight / total sampled weight x 100
Gap to specification = contamination rate - specification target

Inputs explained

Out-throw and prohibitive weight in sample:
Total sampled bale or stream weight:
Specification or broker target contamination:

How to use the result

Use it on every QC bale audit, before shipping to a new buyer, or when a mill flags a load.
A single sample only represents the bale it came from; contamination varies bale to bale, so trend multiple samples rather than acting on one.

Current U.S. benchmarks

Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% year over year (Census).

Common questions

How do you calculate contamination rate? Divide the out-throw and prohibitive weight by the total sampled weight, then multiply by 100. With 8 kg of out-throw in a 250 kg sample, the contamination rate is 3.2%.
What is a good contamination rate for a recycled bale? It depends on grade, but many fiber and PET specifications target 5% or lower. The 3.2% in this example sits comfortably 1.8 points under a 5% target, which is a sellable, in-spec result.
What counts as out-throw versus prohibitive material? Out-throws are recyclables of the wrong type that lower grade; prohibitives are items that can damage equipment or render the bale unusable, like trash or hazardous material. Both are summed in the contamination weight here.
What does the gap to specification tell me? It is your contamination rate minus the target. Here 3.2% minus a 5% target gives a gap of 1.8 points to the good side, meaning you have headroom. A positive number toward the target means you are in spec; exceeding the target means rework.
How big should my contamination sample be? Large enough to be representative, typically a full grab or cut from the bale of tens of kilograms. The 250 kg total here is a robust sample; very small samples swing the percentage wildly on a single heavy item.

Last reviewed 2026-05-12.