How much water does 1 mm of rain deliver per hectare?

Exactly 10 cubic metres (10,000 litres = 10 kL = 0.01 ML) per hectare. This is the most important conversion in irrigation and rainfall analysis.

Gross vs effective rainfall?

Gross is total depth in the gauge. Effective is what infiltrates and becomes available to the crop after subtracting runoff and evaporation from leaf surfaces.

How to calculate pump run time?

Volume (L) = Required depth (mm) x Area (m2). Run time (hrs) = Volume / Pump output (L/hr). Example: 20 mm on 8 ha = 1,600,000 L. At 15 L/s (54,000 L/hr): 1,600,000/54,000 = 29.6 hours.

Rainfall / Irrigation Volume — Free Agriculture Calculator

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The rain gauge read 28 mm and the boss wants to know how many megalitres fell across the property before deciding whether to run the pump to the header paddock today.

Rainfall / Irrigation Volume

Water

Paddock area (ha)

Imperial: enter acres

Rainfall / irrigation (mm)

Imperial: enter inches

Application type

Volume (kL) = Area (ha) × Rainfall (mm) × 10 Rule of thumb: 1mm of rain over 1 hectare = 10,000 litres (10 kL)
Examples: 25mm on 100ha = 25,000 kL (25 ML) · 1" on 247ac ≈ same volume
For dam/catchment runoff, multiply by a runoff coefficient (typically 0.3–0.8 depending on slope and soil).

Reference: Bureau of Meteorology — Climate Data · DAFF — Water Resources

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates total water volume from a rainfall event or irrigation system. Enter depth (mm) and area (ha) to get volume in megalitres, cubic metres and kilolitres. Also calculates irrigation run time from pump output and required depth.

2 Formula & professional reasoning

Volume (ML) = Rainfall or depth (mm) x Area (ha) / 10,000
Volume (m3) = Rainfall (mm) x Area (ha) x 0.1
Key: 1 mm on 1 ha = 10 m3 = 10 kL = 0.01 ML

1 mm of rain over 1 hectare (10,000 m2) = exactly 10,000 litres = 10 m3 = 0.01 ML. Dividing mm x ha by 10,000 gives megalitres. For irrigation scheduling, depth tells you soil moisture replenishment while volume tells you whether you are within your water licence allocation.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Rainfall volume across a paddock

Given: Rainfall: 28 mm | Area: 120 ha

Working: Volume: 28 x 120 / 10,000

Answer: 0.336 ML (336,000 L = 336 m3)

💡 A 28 mm rain on 120 ha delivers over 330,000 litres — enough for good germination.

Standard

Irrigation run time calculation

Given: Pump: 15 L/s | Run time: 6 hours | Block: 8 ha

Working: Volume: 15 x 6 x 3,600 = 324,000 L = 0.324 ML | Depth: 324,000 / (8 x 10,000) = 4.05 mm

Answer: 4 mm depth on 8 ha | 0.324 ML consumed

💡 Summer maize transpires 6-8 mm/day in peak heat — 4 mm is sub-maintenance. Check crop demand.

Advanced

Annual irrigation allocation check

Given: Water licence: 800 ML | Target: 6 ML/ha | Area: 120 ha

Working: Required: 120 x 6 = 720 ML | Headroom: 800 - 720 = 80 ML

Answer: 720 ML required | 80 ML headroom (10% buffer)

💡 Tight in a dry year. Consider purchasing temporary water before prices spike.

4 Sanity check

Fundamental conversion to memorise

1 mm on 1 ha = 10 m3 = 10 kL = 0.01 ML

All other calculations scale from this relationship.

Effective vs gross rainfall

Not all rainfall becomes plant-available water

Sandy soils: most infiltrates. Clay after saturation: significant runoff on large events.

Irrigation volume to depth

1 ML per ha = 100 mm depth equivalent

Pump output reference

1 L/s = 3,600 L/hr = 86,400 L/day

5 Common errors

Error	Cause	Consequence	Fix
Area in m2 instead of hectares	Confusion between m2 and ha	Volume off by 10,000x	Area must be in hectares. 1 ha = 10,000 m2.
Confusing ML and kL	Megalitres vs kilolitres	Volume off by 1,000x — wrong licence decision	1 ML = 1,000 kL = 1,000,000 L.
Not accounting for runoff losses	Assuming all rainfall infiltrates	Plant-available water overestimated	Effective rainfall = total minus runoff. Varies by soil and event intensity.
Nameplate pump output instead of measured	Taking flow rate from motor spec	Volume calculated on wrong rate	Measure actual output at operating conditions — typically 10-20% less than nameplate.