Free dew point and condensation calculator

Start with two measurements, not a guess

Enter the temperature and relative humidity measured in the room you are investigating. A weather app usually describes outdoor conditions, which are not a substitute for an indoor reading. Place your instruments according to their instructions and keep them away from direct heat, splashes or other conditions that would distort the result. Record the time if you plan to compare several mornings.

The optional surface field is for a measured window, wall or other accessible surface. Leave it blank if you do not know the temperature. Guessing that the glass is the same temperature as the room defeats the purpose of the comparison. Reflective glass can also be difficult to measure accurately with some infrared instruments; follow the instrument guidance rather than treating every displayed number as exact.

What the dew point tells you

Dew point is the temperature at which the air would reach saturation if cooled at constant pressure. If a surface is at or below that temperature, water can condense there. A room can feel warm while its window remains much colder, which is why comfort alone does not tell you whether the glass will become wet.

The result is rounded to one decimal place for readability. That is not a claim that your instruments or this approximation are accurate to a tenth of a degree. The tool compares the optional surface reading with the calculated threshold. It also marks a practical caution band when the surface is less than 2°C above the dew point. This band is our explanatory margin, not a regulatory limit or a standard for mould prevention.

A worked example: 20°C air and 60% humidity

At 20°C and 60% relative humidity, the calculation gives a dew point of about 12.0°C. A measured surface at 10°C is below that threshold, so condensation is physically possible under those inputs. This is a mathematical example, not a measurement of your home. Use the example button to see the same values in the workspace.

Keeping the air temperature at 20°C while changing humidity to 50% gives a dew point of about 9.3°C. At 70%, it is about 14.4°C. The comparison shows why moisture in the air matters. It does not promise that a particular ventilation habit or appliance will produce either humidity level. The room, weather, building and moisture sources determine what happens in practice.

Use the comparison table to understand a change

The table holds air temperature constant and varies humidity by ten percentage points on either side where possible. Going from 60% to 50% is a change of ten percentage points; it is not the same as reducing the original value by ten percent. Near the ends of the allowed range, the table uses the nearest valid value and removes duplicates.

Try a real before-and-after reading only after making a sensible, safe adjustment, then note what else changed. Outdoor weather, heating and occupancy can affect the comparison. One improved reading is useful information, but it does not prove that a recurring damp problem has been solved. Keep a short dated record if the issue needs professional investigation.

What this calculator does not decide

The model does not inspect a building, detect mould, identify a leak, measure ventilation or recommend a medical treatment. A surface above the calculated dew point now may have been colder earlier or may contain colder spots elsewhere. Water between sealed panes and a wet patch following rain need different investigation from ordinary room-side condensation.

For this reason, the tool does not display a green “your room is safe” certificate. It reports the relationship between the supplied measurements and a physical threshold. If you see persistent damp or mould, follow appropriate property and health guidance. Do not block the building’s intended ventilation or buy equipment solely because a calculator produced an uncomfortable number.

The calculation and its useful range

We use a Magnus-style approximation over liquid water with constants 17.67 and 243.5, consistent with the National Weather Service calculation reference linked below. With air temperature T in Celsius and relative humidity RH, gamma = ln(RH / 100) + 17.67 × T / (243.5 + T), then dew point = 243.5 × gamma / (17.67 − gamma).

This workspace is limited to ordinary indoor air from 0 to 50°C, relative humidity from 1 to 100%, and optional surfaces from 0 to 60°C. It is not designed for industrial processes, frost calculations or high-altitude meteorology. Fahrenheit entries are converted before calculation and results converted back. Switching units preserves the physical meaning of the entered temperatures, subject to displayed rounding.

Turn the number into a useful next step

Use the result alongside a simple observation record: where the water appears, when it appears, what the room readings were and whether cooking, bathing or indoor drying preceded it. A clear record helps you decide what to investigate and gives a property professional better information if the issue persists.

Read our morning window-condensation guide for the full process. You can copy the calculation explanation to your own notes. The tool keeps no history, so closing or reloading the page clears your inputs. No subscription or account is needed, and the calculation does not contact a weather service.

Sources: National Weather Service calculation reference, dew point explanation and EPA moisture guidance.