millimeter of depth
Volume per areaexact by definition
A millimetre of depth is a volume per area: 10⁻³ m³ of water spread over each square metre, exactly 10⁻³ m³/m². It is how rainfall, irrigation and evapotranspiration are all reported in metric practice, and it equals exactly 10,000 litres per hectare.
Why it is written : Written simply "mm", the same symbol as the length, because it IS a length — a depth of water standing on the ground. The site lists it separately from the length millimetre only so the two can be converted against volumes per area.
| 1 | 10,000 |
About the millimeter of depth
This unit is the reason volume per area is worth having as its own type. A cubic metre of water spread evenly over a square metre stands one metre deep, so any volume-per-area is a depth, and the whole vocabulary of irrigation collapses into one number.
1 mm = 10 m³/ha = 10,000 L/ha = 1 L/m².
That single line converts a rainfall figure into a tank volume. A 25 mm rain on a 40-hectare field delivered 10,000 cubic metres of water — ten million litres. And a crop using 6 mm/day on that field needs 2,400 m³ a day from the irrigation system, which is 27.8 litres per second running continuously.
It also explains why a spray application is not a watering. At 200 L/ha, a sprayer applies 0.02 mm. You would have to spray the field 1,250 times to equal one good rain.
One millimeter of depth in every volume per area unit
| 10,000 | liter per hectare (L/ha) | |
| 10 | cubic meter per hectare (m³/ha) | |
| 1,000 | milliliter per square meter (mL/m²) | |
| 1 | millimeter of depth (mm)this unit | |
| 0.1 | centimeter of depth (cm) | |
| 1,069.07 | US gallon per acre (gal/ac) | |
| 24.5424 | US gallon per thousand square feet (gal/1000 ft²) | |
| 0.0393701 | inch of depth (in) |