Dew point calculator: is the steel warm enough to coat?
Enter the air temperature, the relative humidity and the steel temperature. The calculator gives the dew point and says whether the steel meets the margin of the unit you choose: 3 °C or 5 °F.
This tool is a reference aid. Before you act on a result, check it against your specification and the product data sheet.
The rule: blast and coat only when the steel surface is at least 3 °C above the dew point (ISO 8502-4). SSPC-PA 1 sets 5 °F instead, and 3 °C is 5.4 °F, so the two limits differ a little. The calculator applies the limit of the unit you choose: 3 °C in °C, 5 °F in °F. Many specifications also limit relative humidity, often to 85 %. Your specification decides.
How it works
The calculator uses the Magnus-type formula in ISO 8502-4:2017 Annex A, with a = 17.08085 and b = 234.175:
td = b × ((b + t) × ln(RH/100) + a × t) ÷ (a × b − (b + t) × ln(RH/100))t is the air temperature in °C and RH the relative humidity in %. The margin is the steel temperature minus the dew point.
Worked example
Air 18 °C, RH 75 %. The dew point is about 13.5 °C. Steel at 16 °C is 2.5 °C above it: below the 3 °C margin, so do not coat yet. Steel at 17 °C would pass.
Limits
- Below 0 °C the formula is outside its stated range.
- The calculator accepts air from −20 to 60 °C (−4 to 140 °F) and steel from −30 to 80 °C (−22 to 176 °F). With steel at 0 °C or below it warns of ice or frost: do not coat an iced or frosted surface.
- Measure the steel, not the air: a contact thermometer on the surface you will coat, in shade and in sun.
- Conditions change during the shift. Record them at the start, at intervals and whenever the weather turns.
Try it in an exam-style question. The published AMPP CIP Level 1 guide lists relative humidity and dew point, and the FROSIO book teaches the 3 °C margin. The free readiness check shows the question style and explains every answer.
Take the free readiness checkSee the tracksRelated: Dew point, RH and steel temperature guide · Concept: dew point · °C ↔ °F converter.
Sources
- ISO 8502-4:2017 — guidance on the estimation of the probability of condensation (Annex A formula)
- DeFelsko — measuring environmental conditions for the application of protective coatings
Checked on 4 October 2026. CoatMentor is an independent study aid; it is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.