Paint Drying Time Calculator

How the paint drying time calculator works

Every paint maker prints a recoat time on the tin or the data sheet, measured at standard conditions of about 20 to 25°C (68 to 77°F) and 50% relative humidity. That figure is the calculator's answer. What your temperature and humidity decide is something more useful than a slightly adjusted number: whether the conditions allow painting at all. The calculator checks the air against the coating's minimum application temperature, and against the dew point, the temperature at which moisture would condense on the surface.

No site formula turns 12°C into an exact recoat time, and the makers do not publish one. Their data sheets say the same thing in different words: drying and recoat times depend on temperature, humidity and film thickness. So the calculator reports the maker's time, then tells you when conditions put it at risk.

Recoat times from the data sheets

Water-based wall paint recoats quickly. The Sherwin-Williams Emerald Interior data sheet gives touch dry in 1 hour and recoat in 4 at 77°F and 50% RH, and the Dulux Trade Vinyl Matt data sheet gives 2 to 4 hours. Water-based trim paint is similar: the ProClassic waterborne sheet also says 1 hour to touch and 4 to recoat. Oil-based gloss is much slower. The Dulux Trade High Gloss sheet gives touch dry in 4 to 6 hours and recoat in 16 to 24, and the calculator uses the longer end.

Too cold to paint

Each coating has a floor. Dulux, following British Standard BS 6150, says not to apply its water-based paints below 8°C and its oil-based gloss below 5°C. Sherwin-Williams asks for 50°F (10°C) and above for its interior latex paints. The calculator uses the stricter 10°C (50°F) for water-based interior paint and flags anything colder as too cold to apply. It also flags interior paint and plaster work above 38°C (100°F), the top of the range USG says interior paint makers usually give.

Exterior acrylics are made to go on colder, and the Emerald Exterior data sheet shows what the cold costs. At 45°F and above it recoats in 4 hours. Between 35 and 45°F, touch dry doubles to 2 hours and recoat stretches to 24 to 48 hours. Below 35°F (1.7°C) it must not be applied, nor when the temperature may fall below that within 48 hours. The calculator uses that band for exterior and masonry paint, taking the 48-hour end.

For outside work, plan by the overnight low, not the afternoon high. A wall painted at 2 pm in mild sunshine can sit near freezing by dawn, with the film still wet. The same sheet asks painters to avoid rain or snow expected within 2 to 3 hours, so an evening shower rules out a late-afternoon coat even on a warm day.

Humidity and the dew point

Humidity matters most through condensation. The same Sherwin-Williams sheet asks for the air, the surface and the paint to be at least 5°F (2.8°C) above the dew point. The calculator works out the dew point from your temperature and humidity with the Magnus approximation and compares it with the air temperature. A surface can be colder than the air, especially an outside wall early in the morning, so treat a narrow margin as a reason to wait.

Two examples show how quickly the margin closes. At the US default of 65°F and 55% RH, the dew point is 48°F, a comfortable 16.6°F below the air. At 60°F and 90% RH it is 57°F, only 2.9°F below, and the calculator flags it even though the temperature alone would be fine. In metric, 18°C at 55% gives a dew point of 8.8°C, 9.2°C clear.

New plaster before the mist coat

Plaster has no recoat time; it has to dry. The USG guide to preparing plaster for painting says its thin veneer finishes can be dry in as little as 24 hours, notes that interior paint makers typically recommend 50 to 100°F for application, and warns that high humidity prolongs drying. Its test is simple: tape a square foot of clear plastic to the wall and leave it for 24 hours; droplets underneath mean the plaster is still too wet. The calculator allows 72 hours as a planning figure for a skim coat, and the plastic test decides. Gauged-lime putty finishes over conventional backing plaster are slower still: USG asks for 30 days before decorating. The same guide warns against PVA-based primers on any plaster, because they soften when wetted and slow the drying underneath.

What this tool does not do

It does not know your exact product, film thickness or airflow, and it takes the air temperature as the surface temperature. Always check the data sheet for the paint you are using, which wins over any general figure here, and measure the surface with a thermometer on a cold day. Recoat time is not full cure: most paints stay soft for days or weeks, so go gently with furniture, tape and washing.

Using this calculator with other BuildMetricLab tools

The paint calculator works out how much you need, and the build timeline calculator helps fit the drying time between trades. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.