October 11, 2026

How to calculate soil moisture content at home

Soil moisture content is the weight of water in a sample divided by the weight of the same soil once it's dry, multiplied by 100. You find it by weighing a sample wet, drying it, and weighing it again. The result is called gravimetric water content, and it is a different number from the percent your bed sensor shows.

The soil moisture content equation

Write it as moisture content (%) = (wet weight minus dry weight) ÷ dry weight × 100. Drying drives the water out of the pores, so the weight the sample loses is the weight of its water. You divide by the dry weight because the mineral and organic matter stays constant while the water comes and goes.

A second version, volumetric water content, divides the volume of water by the total volume of soil. Most soil sensors and irrigation tables work in volumetric terms. You can get both figures from one sample if you cut it with a container of known volume.

Step 1. Cut a sample of known volume

Push an empty can with both ends removed straight down into the bed until its rim sits flush with the surface. Dig around it, lift it out, and trim the bottom flat with a knife. Write down the can's inside volume. Take the sample at root depth, since the surface dries first and reads drier than the soil your roots draw on.

Step 2. Weigh it wet

Tip the soil into an oven-safe dish you've already weighed, and weigh it right away. Soil starts losing water to the air the moment it leaves the ground, so a sample that sits on the bench for an hour will read low. Subtract the dish weight and write the number down.

Step 3. Dry it in a low oven

Spread the soil thin and dry it at low heat for several hours. Weigh it, put it back, and weigh it again a few hours later. It is dry when two weighings match. Keep the heat low, because high heat burns off organic matter, and that lost mass gets counted as water.

Step 4. Weigh it dry

Weigh the dish again and subtract its weight. The difference between your wet and dry weights is the water the sample held.

Step 5. Run the numbers

Suppose your can holds 100 cubic centimeters, and the sample weighs 160 grams wet and 125 grams dry. The water weighs 35 grams. Gravimetric content is 35 ÷ 125 × 100, which gives 28%. Water weighs one gram per cubic centimeter, so those 35 grams fill 35 cubic centimeters of the can's 100, and volumetric content is 35%. In mineral garden soil the volumetric figure comes out higher, because dry soil packs more weight into each cubic centimeter than water does.

Step 6. Check your bed probe against a sample

If you have a bed probe, cut your sample beside it at the same depth, and note the probe's reading at the same time. Compare that reading with your volumetric figure. Repeat on a wet day and a dry day. The gap tells you how far the probe's factory curve sits from your soil.

Why a sensor percent differs

A capacitive probe such as the Ecowitt WH51 never weighs anything. It measures how well the soil around it stores an electric charge. Water holds charge far better than mineral grains or air, so the reading climbs as the pores fill. The probe turns that signal into a percent with a curve the maker set for a generic soil.

Texture, salt, organic matter, and air gaps against the probe all shift that curve. A clay bed and a compost-heavy raised bed holding the same water can report different numbers. Between mid-July and early September in our own garden, the ground bed ranged across 32% to 62%. Raised bed 1 held between 39% and 54% across those weeks. Those figures compare each bed with itself over time. They don't tell us the gravimetric content of either bed.

A Watermark probe reports a third quantity, soil tension in centibars. Tension measures how hard roots must pull to get water out, so the number rises as the soil dries.

What can go wrong

• Heat too high. Organic matter burns off, and the result reads wetter than the soil was.
• Slow weighing. The sample dries on the bench and reads drier than the bed.
• Stones and roots. A large stone adds dry weight with no pore space, which drags the percent down. Pick stones out before the wet weighing.
• One spot. Beds dry unevenly from edge to center. Take two or three samples and compare them.

The calculation gives you an exact water content for one handful of soil on one morning. It cannot tell you whether your plants are short of water, because sand and clay at the same content hold that water with very different force.

How Siltwell reads your probe

Once your station and a soil probe are connected to Siltwell and the probe is mapped to a bed, Siltwell checks the readings every 10 minutes. When the two most recent readings from a capacitive probe both fall below 25%, it sends a warning alert and a push notification naming the bed. A Watermark probe triggers the same alert when its two latest readings both pass 60 cb. Requiring two readings in a row keeps a single odd value from setting it off.

Every bed uses the 25% line, and you'll get at most one alert per bed per day. Siltwell tells you which bed is dry and leaves the watering to you. It doesn't run your irrigation.

Because the alert works from the probe's own percent, Step 6 pays off here. If your sample shows the probe reads well below your soil's volumetric content, the alert will fire on a bed that holds more water than 25% suggests, and you can wait a day before watering.

To get a probe reporting, start with connecting your station. To decide what a reading should trigger in your soil, read how to set a soil water content threshold and what to measure before you water.