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Any one of the three from the other two.
A hygrometer reads air temperature and relative humidity, and the dew point follows from them. A dew point meter or a weather report gives the dew point, and with the air temperature that gives the humidity. The third way round answers how warm the air would have to be for a given dew point to mean a given humidity.
The dry-bulb temperature of the air where the work is.
Read it out of direct sun and away from heaters, at the height and place of the surface being worked, not at the site cabin.
Tools needed: Thermometer or hygrometer
The hygrometer's reading, in %, taken with the air temperature.
Let the instrument settle for a few minutes where the work is. Wattyl's application conditions sheet notes that most coatings have an upper limit of 85% relative humidity, and Tnemec's guide for its vinyl ester systems says not to apply above 80%; the product's own data sheet has the figure that applies.
Tools needed: Hygrometer
The temperature of the surface to be coated, measured on the surface itself.
Use a contact or surface thermometer on the steel, concrete or timber, at the coldest part to be coated: the shaded side, the base of a wall, the bottom of a tank. The check sets that reading against the dew point of the air around it. For steel, ISO 8502-4 (2017 edition) gives guidance on estimating the probability of condensation on a surface to be painted.
Tools needed: Surface (contact) thermometer
How far above the dew point the surface has to be, from the coating's data sheet: three Celsius (five Fahrenheit) degrees on Wattyl's and Tnemec's.
Wattyl's application conditions sheet: "normally the substrate temperature must be at least 3 °C above the dew point". Tnemec's application guide: "at least 5°F (3°C) above the dew point". It is one rule rounded in each market (three Celsius degrees is 5.4 Fahrenheit degrees), and it is a safety allowance, because a surface only a little above the dew point can still take moisture from the air. Use the figure on your product's own sheet where it differs.
Answer
53.59 °F
The surface stands above the dew point by at least the margin entered, at these readings. That is one condition of a coating's data sheet, not all of them: its own limits on humidity and on air and surface temperature apply as well, the readings move through the day, and the decision to coat is the applicator's.
- Dew point
- 53.59 °F
- Air temperature
- 68 °F
- Relative humidity
- 60 %
- Air temperature above the dew point
- 14.41 Δ°F
- Surface temperature above the dew point
- 10.81 Δ°F
- Lowest surface temperature the coating check asks for
- 58.59 °F
They open the calculator with your figures already in it
Dew Point and Relative Humidity Calculator (with the Coating Surface Check): 53.59 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I (2023 edition), Annex 4.B, Formulae for the computation of measures of humidity: e_w(t) = 6.112 exp[17.62 t / (243.12 + t)], water (pure phase), −45 °C to +60 °C, uncertainty up to 0.6% (k = 2); over ice 6.112 exp[22.46 t / (272.62 + t)], −65 °C to 0 °C. Read as the WMO reproduces the annex in its own training unit, Calibration of humidity instruments, Part 1
- Algebra of the Magnus form with b = 17.62 and c = 243.12 °C, since relative humidity is the vapour pressure over the saturation pressure at the air temperature: dew point Td = cγ / (b − γ) with γ = ln(RH/100) + bT / (c + T); RH = 100 exp(bTd / (c + Td) − bT / (c + T)); air temperature T = cδ / (b − δ) with δ = bTd / (c + Td) − ln(RH/100)
- Wattyl (Valspar Paint (Australia) Pty Ltd), Protective & Marine InfoSheet I-05, Ambient Application Conditions (Version 6, March 2021): to allow a safety margin, normally the substrate temperature must be at least 3 °C above the dew point; most coatings have an upper limit of 85% relative humidity; worked example: 16 °C at 71% relative humidity gives a dew point of 11 °C, and a substrate at 13 °C is only 2 °C above it, so painting should not proceed
- Tnemec, Application Guide SS-5.0 to SS-5.2 (Propolymer and Vinester special systems), 3.3 Dehumidification: the surface should be clean, dry and contaminant free, and be at least 5°F (3°C) above the dew point; do not apply when humidity exceeds 80%
- ISO 8502-4:2017, Preparation of steel substrates before application of paints and related products — Tests for the assessment of surface cleanliness — Part 4: Guidance on the estimation of the probability of condensation prior to paint application (published 9 January 2017), as the Standards Council of Canada records it: it "gives guidance on the estimation of the probability of condensation on a surface to be painted" and "may be used to establish whether conditions at the job site are suitable for painting or not". The standard's text is not reproduced here, and no figure on this page is taken from it
Inputs used
- Work Out
- The dew point, from the air temperature and relative humidity
- Air Temperature
- 68 °F
- Relative Humidity (%)
- 60
- Air Temperature (with a Known Dew Point)
- 68 °F
- Dew Point
- 53.6 °F
- Dew Point (to Find the Air Temperature)
- 53.6 °F
- Relative Humidity (%, to Find the Air Temperature)
- 60
- Surface Temperature (for the Coating Check)
- 64.4 °F
- Margin Above the Dew Point the Coating Asks For
- 5 °F
Intermediate steps
- Dew point
- 53.59 °F
- Air temperature
- 68 °F
- Relative humidity
- 60 %
- Air temperature above the dew point
- 14.41 Δ°F
- Surface temperature above the dew point
- 10.81 Δ°F
- Lowest surface temperature the coating check asks for
- 58.59 °F
Confidence note: The surface stands above the dew point by at least the margin entered, at these readings. That is one condition of a coating's data sheet, not all of them: its own limits on humidity and on air and surface temperature apply as well, the readings move through the day, and the decision to coat is the applicator's.
What this calculation does not cover
- Below freezing this is the dew point over water, the temperature at which supercooled dew would form. Frost forms at the frost point over ice, a little higher, which the WMO works with different constants; a surface below freezing collects frost before this figure says it would collect dew.
- The air reading and the surface reading have to be taken together, where the work is. The air next to a cold steel wall, inside a tank or under a slab is not the air at the door, and a surface in the sun, in shade or against cold ground can sit several degrees from the air around it. A contact thermometer on the surface itself, not an air probe held near it, is what the surface figure should come from.
- Conditions move. The dew point rises as the evening cools the air towards it and falls as a dry wind comes through; a reading that clears the margin at noon may not clear it at dusk, while the coating is still curing. Coating makers ask for the checks to be repeated through the work and during the cure.
- The margin is one condition of a coating's data sheet, not all of them. A product's sheet can also set its own lowest and highest air and surface temperatures and a highest relative humidity, and some products need a lowest humidity to cure; the product's own sheet governs, and the decision to coat is the applicator's.
- The formula is the WMO's Magnus form for water from −45 to 60 °C (−49 to 140 °F), with an uncertainty of up to 0.6% in the vapour pressure, which is about a tenth of a Celsius degree (two tenths of a Fahrenheit degree) in the dew point. Outside that range the figure is shown but marked, because the coefficients were not fitted there.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations5
- WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I (2023 edition), Annex 4.B, Formulae for the computation of measures of humidity: e_w(t) = 6.112 exp[17.62 t / (243.12 + t)], water (pure phase), −45 °C to +60 °C, uncertainty up to 0.6% (k = 2); over ice 6.112 exp[22.46 t / (272.62 + t)], −65 °C to 0 °C. Read as the WMO reproduces the annex in its own training unit, Calibration of humidity instruments, Part 1
- Algebra of the Magnus form with b = 17.62 and c = 243.12 °C, since relative humidity is the vapour pressure over the saturation pressure at the air temperature: dew point Td = cγ / (b − γ) with γ = ln(RH/100) + bT / (c + T); RH = 100 exp(bTd / (c + Td) − bT / (c + T)); air temperature T = cδ / (b − δ) with δ = bTd / (c + Td) − ln(RH/100)
- Wattyl (Valspar Paint (Australia) Pty Ltd), Protective & Marine InfoSheet I-05, Ambient Application Conditions (Version 6, March 2021): to allow a safety margin, normally the substrate temperature must be at least 3 °C above the dew point; most coatings have an upper limit of 85% relative humidity; worked example: 16 °C at 71% relative humidity gives a dew point of 11 °C, and a substrate at 13 °C is only 2 °C above it, so painting should not proceed
- Tnemec, Application Guide SS-5.0 to SS-5.2 (Propolymer and Vinester special systems), 3.3 Dehumidification: the surface should be clean, dry and contaminant free, and be at least 5°F (3°C) above the dew point; do not apply when humidity exceeds 80%
- ISO 8502-4:2017, Preparation of steel substrates before application of paints and related products — Tests for the assessment of surface cleanliness — Part 4: Guidance on the estimation of the probability of condensation prior to paint application (published 9 January 2017), as the Standards Council of Canada records it: it "gives guidance on the estimation of the probability of condensation on a surface to be painted" and "may be used to establish whether conditions at the job site are suitable for painting or not". The standard's text is not reproduced here, and no figure on this page is taken from it
Which documents these citations point at
Standards referenced: ISO 8502-4 (International Organization for Standardization, International).
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