Rh Calculator

The Rh Calculator finds relative humidity from air temperature and dew point. It gives a percent figure that shows how close the air is to saturation at that temperature.

Students, gardeners, and people who work with indoor air use relative humidity to judge comfort, mould risk, and drying conditions. A percent near 100 means the air is close to forming dew or fog.

What the Rh Calculator does

What relative humidity means

Relative humidity compares the water vapour in the air with the maximum vapour the air can hold at that temperature. Warm air can hold more vapour than cold air, so the same vapour amount gives a lower percent on a warm day.

Why dew point matters

Dew point is the temperature at which air becomes saturated and dew starts to form. When dew point is close to air temperature, relative humidity is high. When dew point is far below air temperature, relative humidity is low.

The formula in plain words

It uses a standard approximation that compares vapour pressure at dew point with vapour pressure at air temperature. That ratio is multiplied by 100 to give a percent. This method is widely used for everyday weather and indoor checks.

What the percent tells you

A low percent means dry air, which can feel harsh and dry out skin, wood, and plants. A high percent means moist air, which can feel sticky and raise mould risk indoors. Comfort often sits in the middle range.

How to use the calculator

1. Enter air temperature in degrees Celsius.
2. Enter dew point in degrees Celsius.
3. Check that dew point is not above air temperature.
4. Press Calculate and read the relative humidity percent.
5. Try a second pair of values to compare conditions.
6. Press Reset to clear the fields and start again.

Worked Example 1: A clear case

Inputs

Air temperature 25 degrees Celsius, dew point 18 degrees Celsius.

Calculation

First apply the standard approximation to vapour pressure at 18 degrees and at 25 degrees. The ratio of those two values is about 0.6515. Multiply by 100 to reach the percent.

Result

The relative humidity is 65.15 percent.

Worked Example 2: A clear case

Inputs

Air temperature 30 degrees Celsius, dew point 20 degrees Celsius.

Calculation

First apply the standard approximation to vapour pressure at 20 degrees and at 30 degrees. The ratio of those two values is about 0.5508. Multiply by 100 to reach the percent.

Result

The relative humidity is 55.08 percent.

Practical tips

1. Use the same unit for both temperatures.
2. Measure in the shade for outdoor readings.
3. Check dew point with a reliable sensor.
4. Compare morning and afternoon values.
5. Ventilate a room when indoor humidity stays high.
6. Watch for condensation near cold surfaces.
7. Record the pair of values with each result.

Checking your relative humidity estimate

Begin with air temperature and check that the number matches your latest statement or quote. Change dew point by a small amount and watch how the total moves. Compare the result with a second set of inputs so you can see a realistic range. Review each input once more, because one typed digit can change the whole answer.

Checking your relative humidity estimate

Try a lower value and a higher value to learn which input matters most here. Note the result and the inputs together, so the figure still makes sense later. Adjust air temperature first when the answer looks too high, since it usually drives the total. Check dew point against a trusted source before you use the result for a real decision.

Checking your relative humidity estimate

Use round numbers for a first test, then switch to exact figures for the final check. Enter your numbers slowly and read each label, because similar fields are easy to mix up. Look at the result in context, not alone, and ask whether it fits your situation. Keep a note of the date, because prices, rates, and rules can change over time.

Checking your relative humidity estimate

Test the calculator with a simple case you can check by hand for confidence. Save the two examples in your notes and compare your own case with them. Share the inputs with a trusted adviser when the decision involves a large sum. Plan around a range, not one exact figure, when any input is only an estimate.

Checking your relative humidity estimate

Avoid guessing dew point when a bill, quote, or statement can give you the real number. Double-check any percentage, because a small rate change can shift the result a lot. Record the final result and the reason you chose each input for future reference. Set aside time to rerun the check when your income, costs, or plans change.

Checking your relative humidity estimate

Start from the most certain input and build the rest of the estimate around it. Work through the inputs in order, so you do not skip a field by accident. Round only at the end, because early rounding can hide a meaningful difference. Ask what would change the answer most, then focus your research on that input.

Checking your relative humidity estimate

Begin with air temperature and check that the number matches your latest statement or quote. Change dew point by a small amount and watch how the total moves. Compare the result with a second set of inputs so you can see a realistic range. Review each input once more, because one typed digit can change the whole answer.

Checking your relative humidity estimate

Try a lower value and a higher value to learn which input matters most here. Note the result and the inputs together, so the figure still makes sense later. Adjust air temperature first when the answer looks too high, since it usually drives the total. Check dew point against a trusted source before you use the result for a real decision.

Checking your relative humidity estimate

Use round numbers for a first test, then switch to exact figures for the final check. Enter your numbers slowly and read each label, because similar fields are easy to mix up. Look at the result in context, not alone, and ask whether it fits your situation. Keep a note of the date, because prices, rates, and rules can change over time.

Checking your relative humidity estimate

Test the calculator with a simple case you can check by hand for confidence. Save the two examples in your notes and compare your own case with them. Share the inputs with a trusted adviser when the decision involves a large sum. Plan around a range, not one exact figure, when any input is only an estimate.

Checking your relative humidity estimate

Avoid guessing dew point when a bill, quote, or statement can give you the real number. Double-check any percentage, because a small rate change can shift the result a lot. Record the final result and the reason you chose each input for future reference. Set aside time to rerun the check when your income, costs, or plans change.

Checking your relative humidity estimate

Start from the most certain input and build the rest of the estimate around it. Work through the inputs in order, so you do not skip a field by accident. Round only at the end, because early rounding can hide a meaningful difference. Ask what would change the answer most, then focus your research on that input.

Checking your relative humidity estimate

Begin with air temperature and check that the number matches your latest statement or quote. Change dew point by a small amount and watch how the total moves. Compare the result with a second set of inputs so you can see a realistic range. Review each input once more, because one typed digit can change the whole answer.

Checking your relative humidity estimate

Try a lower value and a higher value to learn which input matters most here. Note the result and the inputs together, so the figure still makes sense later. Adjust air temperature first when the answer looks too high, since it usually drives the total. Check dew point against a trusted source before you use the result for a real decision.

Checking your relative humidity estimate

Use round numbers for a first test, then switch to exact figures for the final check. Enter your numbers slowly and read each label, because similar fields are easy to mix up. Look at the result in context, not alone, and ask whether it fits your situation. Keep a note of the date, because prices, rates, and rules can change over time.

Checking your relative humidity estimate

Test the calculator with a simple case you can check by hand for confidence. Save the two examples in your notes and compare your own case with them. Share the inputs with a trusted adviser when the decision involves a large sum. Plan around a range, not one exact figure, when any input is only an estimate.

Checking your relative humidity estimate

Avoid guessing dew point when a bill, quote, or statement can give you the real number. Double-check any percentage, because a small rate change can shift the result a lot. Record the final result and the reason you chose each input for future reference. Set aside time to rerun the check when your income, costs, or plans change.

Checking your relative humidity estimate

Start from the most certain input and build the rest of the estimate around it. Work through the inputs in order, so you do not skip a field by accident. Round only at the end, because early rounding can hide a meaningful difference. Ask what would change the answer most, then focus your research on that input.

Checking your relative humidity estimate

Begin with air temperature and check that the number matches your latest statement or quote. Change dew point by a small amount and watch how the total moves. Compare the result with a second set of inputs so you can see a realistic range. Review each input once more, because one typed digit can change the whole answer.

Checking your relative humidity estimate

Try a lower value and a higher value to learn which input matters most here. Note the result and the inputs together, so the figure still makes sense later. Adjust air temperature first when the answer looks too high, since it usually drives the total. Check dew point against a trusted source before you use the result for a real decision.

Checking your relative humidity estimate

Use round numbers for a first test, then switch to exact figures for the final check. Enter your numbers slowly and read each label, because similar fields are easy to mix up. Look at the result in context, not alone, and ask whether it fits your situation. Keep a note of the date, because prices, rates, and rules can change over time.

Checking your relative humidity estimate

Test the calculator with a simple case you can check by hand for confidence. Save the two examples in your notes and compare your own case with them. Share the inputs with a trusted adviser when the decision involves a large sum. Plan around a range, not one exact figure, when any input is only an estimate.

Checking your relative humidity estimate

Avoid guessing dew point when a bill, quote, or statement can give you the real number. Double-check any percentage, because a small rate change can shift the result a lot. Record the final result and the reason you chose each input for future reference. Set aside time to rerun the check when your income, costs, or plans change.

Checking your relative humidity estimate

Start from the most certain input and build the rest of the estimate around it. Work through the inputs in order, so you do not skip a field by accident. Round only at the end, because early rounding can hide a meaningful difference. Ask what would change the answer most, then focus your research on that input.

Checking your relative humidity estimate

Begin with air temperature and check that the number matches your latest statement or quote. Change dew point by a small amount and watch how the total moves. Compare the result with a second set of inputs so you can see a realistic range. Review each input once more, because one typed digit can change the whole answer.

Checking your relative humidity estimate

Try a lower value and a higher value to learn which input matters most here. Note the result and the inputs together, so the figure still makes sense later. Adjust air temperature first when the answer looks too high, since it usually drives the total.

Frequently asked questions

General questions

1. What does this calculator find?

It finds relative humidity in percent from air temperature and dew point. It uses a standard approximation.

2. Who should use it?

Students, gardeners, and indoor air users will find it useful. It also helps people who check comfort or mould risk.

3. What inputs do I need?

You need air temperature and dew point, both in degrees Celsius. Each figure is entered in its own field.

4. Is the result exact?

No, the result is an approximation. Real sensors and local conditions can give a slightly different figure.

5. How often should I recalculate?

Recalculate whenever temperature or dew point changes. Morning and afternoon values often differ a lot.

Using the result

6. What if dew point is above air temperature?

The calculator will show an error, because dew point cannot exceed air temperature in this check. Check the two figures and try again.

7. Can I use it for planning?

Yes, it works well for comfort checks and for comparing days. Use a calibrated sensor for a final decision.

8. Does it replace professional advice?

No, it does not replace a weather service or building expert. Treat it as a useful everyday check.

9. Why do two places get different results?

Temperature and dew point differ from place to place. Even a small dew point change can move the percent.

10. What is the most common mistake?

The most common mistake is mixing units, such as Fahrenheit with Celsius. Use Celsius for both inputs here.

Planning and next steps

11. How do I read a high result?

A high result means air close to saturation. Dew, fog, or condensation is more likely in that case.

12. How do I read a low result?

A low result means dry air relative to its capacity. Skin, plants, and wood may dry out faster.

13. Can I compare two scenarios?

Yes, run the calculator twice with different temperature pairs. Compare the percent figures side by side.

14. What units should I use?

Use degrees Celsius for both temperatures. The result is a percent, with no unit attached.

15. Where do I start if I am unsure?

Start with a trusted temperature and dew point pair. Check the result against a weather report for the same time.

CONCLUSION

The Rh Calculator gives you a clear starting figure for relative humidity planning. Enter careful inputs, check the result against a second case, and use the figure to guide your next step with confidence.