How to Calculate Rainfall: DIY Rain Gauge Method

This guide shows you how to calculate rainfall depth using a simple, homemade rain gauge. You’ll learn the standard measurement formula, how to take accurate readings, and how to convert your results to the units you need. No special equipment is required beyond a straight-sided container and a ruler.

What You’ll Need

• A straight-sided cylindrical container (e.g., a coffee can, a tall glass, or a plastic bottle with the top cut off). The opening must be the same size as the base.
• A ruler with millimeter or inch markings.
• A flat, level surface outdoors, away from buildings and trees.
• Optional: a funnel and a smaller inner container if you want to reduce evaporation.
• Optional: a marker and tape to mark water levels if you can’t measure immediately.

Step-by-Step

1. Prepare your container. Ensure it is clean and dry. If using a bottle, cut off the top to create a straight, open cylinder. The sides must be vertical so that the area of the opening equals the area of the bottom.
2. Place the gauge in an open area. Choose a spot that is not under trees, eaves, or near walls. It should be on level ground, and the rim of the container should be horizontal. If necessary, use stones or bricks to level it.
3. Wait for the rain event. Leave the container outside for the entire duration of the rainfall. If you plan to measure a single storm, empty it after each reading. For daily totals, check it at the same time each day.
4. Read the water level. After the rain stops, look at the water level in the container. Place the ruler vertically inside, with the zero mark at the bottom of the container. Read the depth of the water at the bottom of the meniscus (the curved surface of the water). Record this value in millimeters or inches.
5. Convert to rainfall depth. Because your container has straight sides, the depth of water you measured is exactly the rainfall depth. If you used a funnel, you must divide the measured depth by the funnel’s collection area factor (see Tips).
6. Record and empty. Note the measurement in a log. Empty the container and dry it before the next rain event.

Common Mistakes

• Using a tapered container. A container that is wider at the top than the bottom will give a false reading because the water level rises faster than the actual rainfall depth.
• Measuring the meniscus incorrectly. Reading the top of the water curve instead of the bottom can add a millimeter or more of error.
• Placing the gauge too close to obstacles. Rain splashing off a roof or being blocked by a tree will skew your numbers.
• Not leveling the container. A tilted gauge catches less rain on one side and more on the other, giving an average that is too low.
• Forgetting to empty after each reading. If you leave water in the gauge, it will evaporate, and you’ll underestimate the next storm’s total.

Tips

• Use a funnel for accuracy. Place a funnel in the top of a narrow inner container. The funnel catches rain from a larger area, amplifying the water level. To get the true rainfall, divide the measured depth by the ratio of the funnel’s opening area to the inner container’s opening area. For example, if the funnel is 10 cm wide and the inner tube is 2 cm wide, the ratio is (10/2)² = 25, so divide the measured depth by 25.
• Measure immediately. The longer water sits, the more evaporates, especially in warm, windy weather.
• Use metric for precision. Millimeters are the standard for rainfall measurement. One millimeter of rain equals one liter per square meter.
• Check for debris. Leaves or insects floating in the water can cause splashing and inaccurate readings. Strain the water before measuring if needed.
• Take multiple readings. If you have space, set up two or three gauges in different parts of your yard and average the results for a more representative total.

Troubleshooting

Reading seems too high. Check that the container is truly vertical and that the bottom is flat. If you used a funnel, double-check your area ratio calculation. Also, ensure no water is being splashed in from a nearby hard surface.

Reading seems too low. The gauge may be sheltered by a nearby wall or overhanging branch. Move it to a more open spot. Also, check for leaks or cracks in the container.

Water level is hard to read. Add a few drops of food coloring to the container before the rain; it makes the water surface easier to see. Alternatively, use a ruler with a dark background.

It rained while you were away. If the gauge overflowed, you can still estimate the amount by measuring the total capacity of the container and noting the time you were away, but this is imprecise. For better results, use a larger container or empty it more frequently.

Frequently Asked Questions

Q: What is the standard unit for rainfall?
A: Meteorologists typically use millimeters (mm) for rainfall depth. One millimeter of rain means that if you had a flat, impermeable surface, the water would be 1 mm deep.

Q: Can I use a regular drinking glass?
A: Yes, as long as it has straight sides (not tapered). A tall, narrow glass works well because it makes small amounts of rain easier to measure.

Q: How do I measure snow with the same gauge?
A: Bring the gauge inside and let the snow melt, then measure the water depth. The general rule is that 10 cm of snow melts to about 1 cm of water, but this varies with snow density.

Q: Why does my reading differ from the local weather station?
A: Rainfall can vary significantly over short distances due to wind and localized storms. Also, official gauges are placed at a standard height and use specific equipment, so differences of a few tenths of a millimeter are normal.

Q: How often should I empty the gauge?
A: After every storm or at least once a day. If you want to track daily totals, empty it at the same time each morning.

Q: What if my container overflows?
A: Use a larger container or empty it more frequently. If it overflows, you lose the data for that period. A 5-gallon bucket can handle most storms, but a tall, narrow cylinder is better for small amounts.

Related

• Inches of Rain: How Rainfall Is Measured
• Snow Water Equivalent Calculator (Snow to Rain)
• Growing Degree Days Calculator (GDD)