Humidity Calculator: Relative Humidity (RH) and Dew Point

Use the Humidity Calculator

Enter temperature and either relative humidity or dew point to get RH, dew point, and practical comfort guidance.

How to Use Humidity Calculator: Relative Humidity (RH) and Dew Point

1. Step 1: Select units

   Choose Celsius (°C) or Fahrenheit (°F) for temperature.

2. Step 2: Choose input type

   Pick “I have RH” or “I have Dew Point,” depending on what you know.

3. Step 3: Enter values

   Type indoor air temperature and either relative humidity (%) or dew point.

4. Step 4: Optional pressure

   Add barometric pressure (hPa) to compute specific humidity (g/kg).

5. Step 5: Review results

   See RH, dew point, absolute/specific humidity, and comfort guidance.

Key Features

• Relative humidity (RH) calculation
• Dew point estimation
• Absolute and specific humidity
• Comfort and condensation cues

Understanding Results

Formula

The calculator uses established meteorological relationships to connect temperature, relative humidity (%RH), and dew point. We estimate the saturation vapor pressure over liquid water with a Magnus‑Tetens expression and compute the actual vapor pressure from %RH. Inverting the same relationship yields the dew point—the temperature where the air would just reach saturation and moisture would start to condense.

Absolute humidity (g/m³) expresses how much water vapor is present in a cubic meter of air, while specific humidity (g/kg) compares the mass of water to the mass of moist air and depends on barometric pressure. These measures change less dramatically with temperature swings and complement %RH for planning ventilation, dehumidification, or comfort settings.

Reference Ranges & Interpretation

For living spaces near 20–24°C (68–75°F), many people find 30–50% RH comfortable. At 50–60% RH, rooms can feel stuffy or heavy but are still acceptable short‑term. Above about 60% RH for extended periods, surfaces and corners may be more prone to mold growth. Dew point is equally useful: around 10–16°C (50–60°F) tends to feel comfortable; above ~18–19°C (64–66°F) feels humid indoors.

Condensation occurs when a surface cools to the dew point or below. If your glass or wall temperature is close to the reported dew point, you may see fogging or droplets—reduce indoor moisture in the evening, keep airflow over cool surfaces, and use exhaust fans in kitchens and bathrooms.

Assumptions & Limitations

Results are for information only and depend on accurate inputs. Sensors and thermostats vary in precision. The formulas assume typical indoor conditions over liquid water; for icy surfaces or extreme environments, specialized models are used. Our tool does not assess health conditions and is not a medical device—use it to plan comfort and moisture control, and consult professionals for mold remediation or building envelope issues.

Complete Guide: Humidity Calculator: Relative Humidity (RH) and Dew Point

Use our humidity calculator to find relative humidity and dew point quickly. See indoor comfort ranges, mold and condensation risks, and ventilation tips.

This guide explains humidity in plain language and shows how to use the tool to answer practical questions: Will my room feel muggy tonight? Is my newborn’s room too dry? Could I get window condensation in the morning? You’ll also learn how dew point relates to comfort and why relative humidity (%RH) changes when temperature changes.

What is relative humidity?

Relative humidity is the percentage of water vapor in the air compared with the maximum the air could hold at the same temperature. Warm air can “hold” more moisture than cool air, so %RH changes even if the total amount of water in the room stays the same. As a rule of thumb, when you heat a closed room, relative humidity drops; when you cool it, relative humidity rises.

Because it depends on temperature, %RH can be misleading on its own. The absolute moisture might be unchanged while %RH moves up or down. That’s why our calculator pairs %RH with dew point and absolute humidity, which better reflect true moisture content.

What is dew point?

Dew point is the temperature at which air becomes saturated and water starts to condense. If indoor surfaces (like window glass) cool to the dew point or below, you’ll see fogging or droplets. Many people find dew points below about 10–12°C (50–54°F) feel crisp or dry, 13–16°C (55–60°F) feel comfortable, 17–18°C (62–65°F) feel humid, and above 19–21°C (66–70°F) feel muggy or oppressive. These are comfort ranges, not medical standards.

Unlike relative humidity, dew point does not change when the temperature changes—unless moisture is added or removed. That makes dew point a reliable way to describe how humid the air feels.

How to use the humidity calculator

The calculator supports two quick workflows. If you have air temperature and relative humidity (from a thermostat, sensor, or a weather app), select “I have RH” and enter the values. The tool outputs dew point, absolute humidity (g/m³), and a comfort cue. If you know the dew point (for example, from a local forecast) and the indoor temperature, select “I have Dew Point” and the tool computes relative humidity and the same moisture metrics.

For a deeper dive, enable the optional pressure field (hPa). That lets the tool estimate specific humidity (g/kg), which is common in engineering and HVAC design. If you don’t know your station pressure, leave the default sea‑level value.

Need temperature conversions before you start? Use our temperature converter. If you’re planning outdoor activity on hot days, check the heat index calculator to see how humidity amplifies heat stress. Sun safety pairs well with the UV index calculator. In winter, comfort outdoors is shaped by wind, so try the wind chill calculator.

Formulas we use

Our calculations follow well‑accepted meteorological relationships. The saturation vapor pressure over liquid water is estimated with a Magnus‑Tetens expression: e_s(T) ≈ 6.112·exp[(a·T)/(b+T)], where T is temperature in °C, a ≈ 17.62 and b ≈ 243.12. Given relative humidity (RH), the actual vapor pressure is e = RH/100 · e_s(T). We then invert the same relationship to find the dew point, using e = e_s(T_d), which yields T_d = b·γ/(a − γ), where γ = ln(e/6.112).

Absolute humidity expresses the mass of water vapor per volume of air. We compute it as ρ_v ≈ 216.7·e/T_K, with e in hPa and temperature in kelvins. Specific humidity (g/kg) depends on total air pressure p (hPa): q = 0.622·e/(p − 0.378·e). Both metrics complement relative humidity because they reflect actual moisture content, which does not swing with temperature as %RH does.

These formulas are appropriate for typical indoor conditions and most everyday weather ranges. For icy surfaces or extreme environments, professional references use ice‑specific constants or more complex saturation models. For everyday planning, the simple formulations are accurate and fast.

Comfort ranges and mold risk

Many homes feel comfortable in the 30–50% RH range when air temperature is around 20–24°C (68–75°F). At 50–60% RH, air may feel a bit heavy or stuffy but still acceptable for short periods. Above ~60% RH for sustained periods, indoor spaces can support mold growth on cool surfaces and in unventilated corners. Keep bathrooms and kitchens ventilated during and after moisture‑creating activities.

Dew point is equally useful for comfort. Many people find dew points near 10–16°C (50–60°F) comfortable indoors. When dew point climbs above ~18–19°C (64–66°F), rooms can feel humid even if temperature is moderate. If you need to hydrate during hot, humid days, see our hydration calculator and daily water intake calculator.

Condensation and window fogging

Condensation happens when a surface is at or below the dew point of the surrounding air. Windows and exterior walls are common cool surfaces. If your calculator result shows a dew point of 12°C (54°F) and your window interior drops to 10°C (50°F) overnight, you’ll likely see fogging or droplets by morning. Improving insulation, increasing air circulation, and reducing indoor moisture during evening hours can help.

Kitchens and bathrooms are frequent culprits. Use exhaust fans that vent outdoors and run them long enough to clear moisture. Keep shower doors open after use, squeegee tiles, and wipe window sills where condensation appears.

Absolute vs. specific humidity

Absolute humidity (g/m³) depends on temperature and pressure indirectly through vapor pressure. It is great for comparing “how much water is in this room” today versus yesterday. Specific humidity (g/kg) compares the mass of water to the mass of moist air and is commonly used in HVAC engineering because it stays consistent as air moves through systems at different pressures. Our tool estimates both, with specific humidity calculated when pressure is provided (a default sea‑level value is used otherwise).

If you are comparing seasons or planning dehumidifier capacity, specific humidity can be more stable than %RH. Pair these insights with broader environmental context using the air quality calculator around wildfire smoke days or smog alerts.

Home ventilation and dehumidifying tips

Keep moisture sources in check first: cover pots while cooking, run a kitchen extractor, and use bathroom fans during and after showers. Dry laundry outdoors when possible. In basements, a dedicated dehumidifier with a drain line can maintain 45–50% RH without constant tank emptying. If rooms feel dry in winter, a small humidifier can improve comfort—monitor %RH to avoid overshooting 50%.

Consider simple airflow fixes: leave doors ajar between rooms, aim fans to mix warm air near ceilings with cooler air below, and avoid blocking vents. In hot climates, scheduling cooling to pre‑dry the air earlier in the evening may reduce overnight condensation. If mold appears, address moisture and clean carefully; for large or hidden growth, consult professionals.

Quick answers (FAQs)

How accurate is this humidity calculator? It uses the widely adopted Magnus approach along with standard conversions to dew point, absolute humidity, and specific humidity. Accuracy is appropriate for home and planning use. Can I use outdoor dew point with indoor temperature? Yes—enter the outdoor dew point with your indoor temperature to estimate indoor %RH when windows are open and indoor and outdoor air mix freely.

References

• National Weather Service (NWS). Heat Index and humidity basics. NOAA JetStream
• U.S. CDC. Mold and dampness, prevention basics. cdc.gov/mold

Worked examples using the tool

Example 1 — Winter dryness: Your thermostat reads 21°C (70°F) and your hygrometer shows 28% RH. Enter 21°C and 28% RH with “I have RH.” The calculator returns a dew point around −1°C (30°F). That’s a very dry indoor dew point. If your skin feels dry or your sinuses are irritated, consider humidifying to reach about 35–40% RH. Notice that if you warm the room to 23°C (73°F) without adding water, RH will drop even further because warm air can hold more moisture.

Example 2 — Summer mugginess: Your room is 25°C (77°F) with 65% RH after cooking. Enter 25°C and 65% RH. The calculator yields a dew point around 18°C (64°F) and absolute humidity near 16 g/m³. That explains the muggy feel. Use the kitchen extractor and, if needed, a dehumidifier to bring RH back to 45–50%. Notice how an evening cool‑down without ventilation can increase RH as temperature falls toward the dew point.

Example 3 — Using dew point from the forecast: Outdoors the dew point is 17°C (63°F). You cool your bedroom to 22°C (72°F) before sleeping. Select “I have Dew Point” and enter 22°C and 17°C. The calculator estimates indoor RH around 56% if you open windows or the air mixes. If the windows fog in the morning, it means the interior glass got close to 17°C—lower indoor moisture before bed to avoid condensation.

Seasonal strategies: winter dryness and summer humidity

In winter, outside air holds less moisture. As that air is heated indoors, relative humidity plummets, which can dry skin and airways. Aim for roughly 35–45% RH to balance comfort and window condensation risk. Raise humidity gradually to find a sweet spot where glass stays clear and the room feels comfortable. In summer, hot outdoor air can contain a lot of moisture. Cooling reduces the air’s capacity to hold water, so RH may rise unless your system actively dehumidifies. Use targeted ventilation during cooking and showers, and consider a standalone dehumidifier where AC alone cannot keep RH below ~55%.

Basements, bathrooms, and kitchens

Basements often sit near ground temperature and can trap moisture. Even modest dew points can lead to high RH against cool walls. Keep air moving with fans, seal obvious water entry points, and run a dehumidifier to target 45–50% RH. Bathrooms and kitchens add moisture quickly; use exhaust fans that vent outdoors and let them run several minutes after finishing a shower or cooking. Wipe damp surfaces to deny mold the water it needs.

Instruments, storage, and sensitive items

Musical instruments, wooden furniture, books, and electronics can suffer when humidity swings. For acoustic guitars, many makers suggest keeping RH near 40–50%. Use the calculator to understand seasonal dew points and how your room’s temperature affects %RH. A small room humidifier or dehumidifier can stabilize conditions. Avoid storing sensitive items directly against exterior walls where temperatures fluctuate and condensation can occur.

Common questions and myths

Myth: “If %RH is the same today and tomorrow, humidity feels the same.” Not necessarily—if temperature changes, the same %RH can feel different. Dew point is a better comfort guide. Myth: “Condensation means a leak.” Sometimes, but often it simply means interior surfaces cooled to the dew point. Improve insulation and airflow, and manage indoor moisture.

Troubleshooting checklist

• Verify readings with a second sensor; calibrate if your device allows it.
• Check for hidden moisture sources: drying laundry, houseplants, aquariums.
• Run exhaust fans long enough; confirm they vent outdoors, not into an attic.
• Increase airflow over cool surfaces and unclog window weep holes.
• Use a dehumidifier with a drain in basements to maintain 45–50% RH.

When to seek professional help

If you see persistent mold, musty odors, water staining, or rot, consider a professional inspection. Building‑envelope issues, roof leaks, or sub‑slab moisture can require specialized fixes. The calculator helps you measure and understand moisture trends, but remediation plans should follow local codes and expert guidance.

Notes on sensors and accuracy

Inexpensive hygrometers can read ±3–5% RH, and response time varies. Place sensors away from direct sun, vents, and exterior walls. For consistent tracking, measure at the same height and location daily. If your device supports calibration, compare it against a reference salt test or a higher‑quality meter.

Data logging and trends

Trends often matter more than a single reading. Logging temperature and humidity helps you correlate spikes with activities—showers, cooking, or laundry—and with weather patterns. Use the humidity calculator to translate those logs into dew point and absolute humidity, which can clarify whether moisture is truly accumulating or if %RH is moving mainly because temperature changed.

Frequently Asked Questions