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WFT ↔ DFT: the wet and dry film thickness formula, with a calculator

By a certified coating inspector · Published 2 October 2026

Specifications set a dry film thickness (DFT), but while the paint is going on, the only thickness anyone can measure is the wet film (WFT). What links the two is the paint's volume solids, adjusted for any thinner. Get the conversion right and the applicator can correct a coat while it is still wet; get it wrong and the error only shows up after cure.

WFT ↔ DFT calculator

The formula

Volume solids is the share of the paint's volume that stays on the surface as dry film; the rest is solvent that evaporates. Paint makers print it on the product data sheet. A film applied wet therefore dries down to that share of its thickness:

DFT = WFT × VS% ÷ 100 WFT = DFT × 100 ÷ VS%

Thinner adds volume but no solids, so it lowers the volume solids of the paint as it is applied:

VS as applied = VS% × 100 ÷ (100 + thinner%) WFT = DFT × (100 + thinner%) ÷ VS% DFT = WFT × VS% ÷ (100 + thinner%)

Thinner % is the volume of thinner per 100 volumes of paint: 1 litre of thinner in 20 litres of paint is 5 %. The units cancel out, so the same formulas work in µm or in mils (1 mil = 0.001 inch = 25.4 µm).

Worked examples

1. The WFT to aim for, no thinner

The specification calls for 150 µm DFT; the paint has 60 % volume solids.

WFT = 150 × 100 ÷ 60 = 250 µm

2. The same paint, thinned 10 %

VS as applied = 60 × 100 ÷ 110 = 54.5 % WFT = 150 × 110 ÷ 60 = 275 µm

The thinned paint has to go on 25 µm thicker to leave the same 150 µm of dry film.

3. Checking a wet reading

A comb shows 300 µm wet on a paint with 75 % volume solids, thinned 5 %.

DFT = 300 × 75 ÷ 105 ≈ 214 µm

4. In mils

5 mils DFT, 68 % volume solids, 10 % thinner.

WFT = 5 × 110 ÷ 68 ≈ 8.1 mils (127 µm DFT → about 205 µm wet)

WFT vs DFT: what each one is for

• WFT is measured on the fresh coat, straight after application. It is a control: ASTM D4414 points out that measuring at the time of application lets the applicator correct the film then, while correcting it after drying or curing costs extra labour and can bring contamination and adhesion problems.
• DFT is measured on the cured film with a magnetic or eddy-current gauge, and it is the number the specification accepts or rejects, under rules such as SSPC-PA 2 or ISO 19840. DFT measurement: SSPC-PA 2 vs ISO 19840 covers those rules.

So a specification is written in DFT, and WFT is how you steer toward it while the paint is still wet.

Reading a wet-film comb

1. Use a clean comb (notch gauge) with undamaged teeth, straight after application, before solvent loss and drying change the film.
2. Press it squarely into the wet film so the teeth stand at right angles to the surface, give the paint a moment to wet the teeth, then lift it off.
3. Read it: the wet film lies between the highest tooth that is wetted and the next one that is dry. ISO 2808 takes the gap of the highest wetted tooth as the reading.
4. On a pipe or any surface curved in one direction, place the comb along the length, parallel to the axis of curvature, not across it.

A wheel gauge, rolled through the wet film, is the other common tool; ISO 2808 describes both. Two limits apply to the comb. ASTM D4414 calls notch-gauge readings neither accurate nor sensitive, useful for an approximate wet film where more precise methods cannot be used. And DeFelsko warns that on quick-drying coatings the readings are inaccurate.

When the formula and the film disagree

• Unrecorded thinner. The same 250 µm wet film from example 1 leaves 150 µm dry unthinned, but only 136 µm if the paint was thinned 10 % (250 × 60 ÷ 110). Record the thinner actually added, or the WFT target is wrong.
• Pigment under the teeth. ISO 2808 notes that every mechanical wet-film method reads low, by at least the mean diameter of the pigment particles trapped between gauge and substrate.
• Some flat coatings. ASTM D4414 notes that with some flat coatings the dry film comes out thicker than the wet reading predicts, which is why a notch reading is not to be used to check a paint's non-volatile content.

In the exams

The AMPP CIP Level 1 exam guide names this skill in its Coatings and Inspection domain, 20 % of the theory exam: using a wet-film gauge and calculating the resulting dry film thickness. The Level 1 practical exam has a wet-film thickness gauge station as well. The same relationship is one step of the longer paint consumption calculation, which goes on from film thickness to spreading rates, losses and the litres to order. Two slips to avoid: leaving the thinner out of the WFT, and adding it to the order quantity, where it does not belong.

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Sources

• Dulux Protective Coatings — Tech Note 5.3.1: Handy calculations, volume solids and spreading rate (PDF) — volume solids, WFT from DFT, the thinner correction, theoretical spreading rate
• ASTM D4414 — Standard Practice for Measurement of Wet Film Thickness by Notch Gages — scope and significance: precision of notch gauges, measuring at application, flat coatings
• ISO 2808:2019 — Paints and varnishes — Determination of film thickness — comb and wheel gauge methods, measuring directly after application, curved surfaces, the pigment-particle error
• DeFelsko — How is coating thickness measured? — using a wet-film comb, curved surfaces, quick-drying coatings
• AMPP — Exam Preparation Guides: Basic Coatings Inspector (CIP Level 1) — the WFT-to-DFT calculation in the Coatings and Inspection domain, the wet-film thickness station in the practical exam

Checked on 2 October 2026.

Related guides: Paint consumption calculation: formula and worked example · DFT measurement: SSPC-PA 2 vs ISO 19840 · Dew point, RH and steel temperature · How to get NACE (AMPP) CIP Level 1.

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