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The FROSIO calculation question, worked end-to-end

By a certified coating inspector · Published 3 July 2026

Unlike the AMPP CIP exams, the FROSIO theory exam is open/written, and it typically ends with a longer paint-consumption calculation that catches people out — not because the maths is hard, but because the terms get mixed up under time pressure. Here is the whole method, worked with a full example.

Note: the method below (loss factor, consumption factor, the WFT formula) is cross-checked against an established coatings-training provider's own calculation approach. Minor conventions — especially the rounding rule and whether volume solids is quoted thinned or unthinned — can still vary by training provider, so confirm against your own course's formula sheet. The numbers below are an original worked example, not a copied exam question.

The quantities you'll juggle

• DFT — dry film thickness, the spec target (µm).
• WFT — wet film thickness, what you apply to land that DFT (µm).
• VS — volume solids of the product, % (the share that stays behind as film).
• Thinner % — how much thinner is added.
• Loss factor — the fraction of paint that actually ends up on the steel, after overspray, wastage and dead volume. Loss factor = (100 − loss%) / 100.
• Consumption factor — simply 1 / loss factor. (Mixing these two up is the single most common mistake.)
• Theoretical spreading rate (TSR) — how far 1 litre goes at the target DFT with zero losses (m²/L).
• Practical spreading rate (PSR) — TSR after losses.

The four formulas

1. Theoretical spreading rate: TSR (m²/L) = 10 × VS% ÷ DFT(µm)
2. Wet film to apply: WFT = DFT × (100 + thinner%) ÷ VS%
3. Practical spreading rate: PSR = TSR × loss factor
4. Paint needed: Volume = Area ÷ PSR, then round up to the supplier's pack size (commonly 20 L pails, as in the worked example below).

Worked example

Apply one coat to 600 m² of steel. Spec DFT 200 µm. Product volume solids 70%, thinned 10%. Allow 30% total losses.

Step 1 — Theoretical spreading rate

TSR = 10 × 70 ÷ 200 = 3.5 m²/L

One litre covers 3.5 m² at 200 µm — if nothing were lost.

Step 2 — Wet film thickness to apply

WFT = 200 × (100 + 10) ÷ 70 = 200 × 110 ÷ 70 ≈ 314 µm

You set your wet-film comb to land ~314 µm wet so that, after the solvent (and the 10% thinner) flash off, you're left with 200 µm dry.

Step 3 — Practical spreading rate

Loss factor = (100 − 30) ÷ 100 = 0.70 PSR = 3.5 × 0.70 = 2.45 m²/L

Step 4 — Paint needed

Volume = 600 ÷ 2.45 ≈ 244.9 L → round up to pack size → 260 L (13 × 20 L)

Cross-check with the consumption factor

Consumption factor = 1 ÷ 0.70 ≈ 1.43 Theoretical volume = 600 ÷ 3.5 = 171.4 L Practical volume = 171.4 × 1.43 ≈ 244.9 L ✓ — same answer, both routes.

The trap almost everyone falls into

Thinner changes your WFT, not your order quantity. Notice that the 10% thinning appears in Step 2 (the wet film you apply) but nowhere in the paint volume you buy. That's because consumption is driven by solids — the dry film you need — and thinner adds no solids. If you inflate the procurement figure by the thinning percentage, you'll over-order every time.

The other classic slip is loss factor vs consumption factor: you multiply theoretical spreading rate by the loss factor (a number below 1), or you multiply theoretical volume by the consumption factor (a number above 1). Reach for the wrong one and your answer lands in the wrong direction.

How to actually get fast at this

The only cure is reps with the method, not re-reading it. Work several examples end-to-end until the four steps are automatic and you can spot, mid-question, which factor goes where — and remember to do it per coat and sum for a multi-coat system.

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Related guide: AMPP CIP Level 1 vs Level 2: which should you take first?

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