Calculate Rain in cm: Step-by-Step Guide

This guide shows you how to measure rainfall and convert it to centimeters (cm) using a standard rain gauge or a simple homemade container. You’ll learn the correct way to read the gauge, convert from millimeters to centimeters, and avoid common errors that skew your data. No special equipment is needed beyond what you likely have at home.

Formula

The core conversion is simple: 1 centimeter = 10 millimeters. So, to convert rainfall from millimeters to centimeters, divide the depth in millimeters by 10.

In symbols: cm = mm / 10

Where:

• cm = rainfall depth in centimeters
• mm = rainfall depth in millimeters (as read from your gauge)

If you measure in inches, convert to cm by multiplying by 2.54 (since 1 inch = 2.54 cm).

Worked Example

Using the default value from the calculator: your rain gauge shows 25 mm of rain.

1. Take the reading: 25 mm.
2. Divide by 10: 25 ÷ 10 = 2.5
3. The result is 2.5 cm of rain.

So, 25 mm = 2.5 cm. The calculator above gives the same answer.

What You’ll Need

• A rain gauge (or a straight-sided container with a flat bottom, like a large jar or a tall can)
• A ruler with millimeter markings (or a fine ruler with 0.5 mm marks)
• A flat, level surface outdoors, away from buildings and trees
• Optional: a funnel (to catch more rain, but then you must adjust the reading—see Tips)
• Optional: a notebook or phone to record readings

Step-by-Step

1. Choose a good location. Place your rain gauge in an open area, away from buildings, trees, or any obstruction that could block rain or cause splashing. The spot should be level and not prone to flooding.
2. Set up the gauge properly. If using a commercial gauge, follow the manufacturer’s instructions. For a homemade container, ensure it is straight-sided and has a flat bottom. Place it on the ground or on a stable post, making sure it is level.
3. Wait for the rain to stop. Check the gauge after a rain event. If you are measuring daily, check at the same time each day (e.g., 9 AM) to get consistent 24-hour totals.
4. Read the water level at eye level. Bend down so your eyes are level with the water surface. Look at the bottom of the meniscus—the curved surface of the water. For most water, the meniscus is concave (dips in the middle), so read the lowest point. If the water is colored or soapy, the meniscus may be convex; then read the highest point.
5. Record the reading in millimeters. Use the ruler to measure the depth of water in the gauge. Most rain gauges have markings in millimeters. If not, place a ruler vertically inside the container and read the water level against it.
6. Convert to centimeters. Divide the millimeter reading by 10. For example, 32 mm ÷ 10 = 3.2 cm. If you have a ruler marked in centimeters, you can read directly, but millimeters give more precision.
7. Empty the gauge. After recording, empty the water and dry the gauge to prepare for the next measurement.

Common Mistakes

• Reading the top of the meniscus instead of the bottom. For water, the meniscus curves down, so reading the top gives a slightly higher value. Always read the lowest point.
• Not using a level surface. If the gauge is tilted, the water depth will be uneven, giving an inaccurate reading.
• Placing the gauge near obstructions. Buildings, trees, and fences can block rain or cause splash, leading to under- or over-estimates.
• Forgetting to convert units. If your ruler is in inches, you must convert to cm (multiply by 2.54) or to mm (multiply by 25.4). Mixing units is a common source of error.
• Measuring too soon after rain stops. Wait a few minutes for the gauge to settle and for any droplets on the sides to run down.

Tips

• Use a gauge with a diameter of at least 4-5 cm (about 2 inches) to reduce evaporation and splashing errors.
• If you use a funnel, the reading will be higher than the actual rainfall because the funnel collects water from a larger area. To correct, divide the reading by the ratio of the funnel’s opening area to the gauge’s tube area. For example, if the funnel opening is 4 times the tube area, divide the reading by 4.
• Check your gauge regularly for debris (leaves, insects) that can block the opening or contaminate the water.
• In snowy conditions, either melt the snow collected in the gauge and measure the water, or use a gauge with a funnel and inner cylinder designed for snow.
• Record your readings in a log to track seasonal patterns and compare with local weather reports.

Troubleshooting

Problem: The reading seems too high or too low.

Check the placement: is the gauge near a sprinkler, a roof overhang, or a tree? Move it to a more open spot. Also ensure the gauge is not overflowing—if the water exceeds the marked scale, you need a larger gauge or you must empty it mid-storm and add the readings.

Problem: Water splashed out of the gauge.

If heavy rain causes splashing, the gauge may lose water. Place the gauge in a hole in the ground so the rim is just above soil level, or use a gauge with a deeper funnel. Alternatively, add a few drops of food coloring to the water to reduce surface tension and minimize splashing.

Problem: The gauge is frozen.

If the water freezes, bring the gauge indoors to melt the ice, then measure the water. Do not try to chip the ice out, as you may lose some.

Problem: The reading is different from local weather reports.

This is normal. Rainfall varies greatly over short distances. Your gauge measures only your specific location. Also, check that your gauge is not being shaded by a tree or building, which can reduce catch.

Frequently Asked Questions

Q: Can I use any container as a rain gauge?

Yes, any straight-sided container with a flat bottom works. A tall, narrow container (like a olive jar) gives more precise readings than a wide, shallow one.

Q: How do I measure rain if I only have a ruler with inches?

Measure the depth in inches, then multiply by 2.54 to get centimeters. For example, 0.5 inches × 2.54 = 1.27 cm.

Q: What is the difference between mm and cm for rain?

They are just different units of the same measurement. 1 cm = 10 mm. Meteorologists often use mm, while gardeners may prefer cm.

Q: Why does my gauge read differently from the weather station?

Rainfall is highly localized. Even a few hundred meters can make a difference. Also, wind can affect catch, so ensure your gauge is not in a windy spot.

Q: How often should I empty my rain gauge?

After every reading. If you expect more rain, empty it to avoid overflow and evaporation.

Q: Can I measure snow with the same gauge?

Yes, but you must melt the snow and measure the water equivalent. Or, use a gauge with a funnel and inner cylinder designed for snow, which typically has a wider opening.

Related

• Snow Water Equivalent Calculator (Snow to Rain)
• Growing Degree Days Calculator (GDD)
• Snow to Rain Calculator: Convert Snowfall to Rain

in d | Rain depth | mm | 25 | 0 | 1000
out cm | Rain in cm | cm | d / 10 | 2
out in | Rain in inches | in | d / 25.4 | 2