📖 How To Use
How to Use This Calculator
Sizing a field irrigation tank correctly prevents both crop stress from under-supply and wasted capital from over-building. Here is the exact process:
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Enter your field area
Measure or look up the irrigated field area in hectares, acres, or square metres. Use only the area that will actually receive irrigation — not the total farm size.
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Select your crop type
Choose from the dropdown. Each option carries its typical peak-season evapotranspiration (ETc) value from the FAO-56 reference. If you have local data from an agricultural extension, select "Custom ETc" and enter it directly in mm/day.
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Pick your irrigation method and efficiency
Drip systems deliver water directly to the root zone and waste very little — typically 90% efficiency. Flood irrigation can lose 40% or more. Select the method closest to yours; or choose "Custom efficiency" to enter a specific percentage.
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Set the refill interval
How often can you refill the tank — from a borehole, river abstraction, or delivery? Enter 1 day if you can refill daily, or the realistic worst-case interval. The tank must hold enough water to bridge the gap between refills.
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Choose a safety buffer
A 10–15% buffer accounts for measurement uncertainty, hotter-than-expected days, and system leaks. Never size a tank to exactly the theoretical minimum — you will run short during peak demand.
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Read your result
The primary figure is the minimum recommended tank capacity in litres. The secondary grid shows the gross water application rate, daily demand in litres, and weight. Use the copy button to save results for procurement.
Tip: This calculator sizes for peak-season demand. If your crop has different ETc across growth stages, enter the peak value for the most critical period (typically mid-season) — it gives you the worst-case tank requirement.
📐 The Formula
Field Irrigation Tank Sizing Formula
The calculation chains together four steps: crop water demand → gross application rate → daily volume per area → total tank capacity required.
Step 1 — Net water need (ETc)
Crop water need (mm/day) = ETc from FAO-56 tables
Step 2 — Gross application rate
Gross (mm/day) = ETc ÷ Irrigation Efficiency
Step 3 — Volume per day
Daily Volume (m³/day) = Gross (mm/day) × Area (ha) × 10
Daily Volume (L/day) = Daily Volume (m³) × 1,000
Step 4 — Tank capacity
Tank (L) = Daily Volume (L/day) × Refill Interval (days) × Safety Buffer
The factor of 10 in Step 3 converts mm/day × hectares into cubic metres per day (1 mm depth over 1 ha = 10 m³).
Irrigation Efficiency Reference
| Method | Typical Efficiency | Water Lost | Best For |
|---|
| Drip | 85–95% | 5–15% | Row crops, orchards |
| Micro-spray | 75–85% | 15–25% | Orchards, soft fruit |
| Sprinkler | 70–80% | 20–30% | Grain crops, pasture |
| Furrow | 55–70% | 30–45% | Row crops on slopes |
| Flood / Basin | 50–70% | 30–50% | Rice, flat fields |
Typical Crop Water Needs (ETc, Peak Season)
| Crop | ETc (mm/day) | Season | Source |
|---|
| Wheat / Barley | 3–5 | Spring–Summer | FAO-56 |
| Maize / Corn | 4–6 | Summer | FAO-56 |
| Rice (paddy) | 4–7 | Summer | FAO-56 |
| Tomatoes | 3–5 | Summer | FAO-56 |
| Cotton | 4–6 | Summer | FAO-56 |
| Sugarcane | 5–8 | Year-round | FAO-56 |
| Citrus / Orchard | 4–6 | Summer | FAO-56 |
| Alfalfa | 5–8 | Summer | FAO-56 |
| Pasture / Grass | 4–6 | Summer | FAO-56 |
| Potatoes | 2.5–4.5 | Summer | FAO-56 |
🌾 When To Use
Use Cases for Field Irrigation Tank Sizing
New Farm Development
When establishing irrigation infrastructure from scratch, tank capacity is one of the first calculations you need. Undersizing forces costly tank upgrades within the first season; oversizing wastes capital that could go toward better pumps or filtration. This calculator gives you a defensible number to take to a supplier or civil engineer.
Upgrading from Open-Channel to Pressurised Systems
Farms switching from flood irrigation to drip or sprinkler systems need to re-evaluate storage. Although efficiency improves — meaning you need less water overall — the pressurised system cannot tolerate interruptions. A buffer tank becomes essential to absorb refill delays and maintain constant pressure at the emitters.
Off-Grid and Remote Irrigation
Fields far from reliable water supply (river abstraction, borehole, or municipal connection) need larger tanks to bridge longer refill intervals. Enter the realistic worst-case interval — not the average — and add a 20–25% buffer. Running dry for even one day during pollination or tuber-fill can wipe out significant yield.
Rainwater Harvesting Integration
If your irrigation tank doubles as a rainwater collection point, size it to cover the longest dry spell between rainfall events, not just refill intervals. Pair this tool with our Annual Rainwater Collection Calculator to understand how much your catchment actually delivers.
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Drip Irrigation
Use a higher efficiency (90%) and smaller tank — but never skip the safety buffer. Emitters clog, pressure drops, and flow rates vary.
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Sprinkler Systems
Wind drift and evaporation during spraying are the biggest losses. Irrigate at dawn or dusk to cut evaporative losses by 15–30%.
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Flood / Furrow
Efficiency of 55–65% means you need roughly 50% more tank volume than a drip system for the same field and crop. Plan accordingly.
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Multi-Day Intervals
For weekly refills, tank size grows linearly — 7× the daily demand plus buffer. Explore underground tanks if above-ground volume becomes impractical.
❓ FAQ
Frequently Asked Questions
How do I calculate the water tank size for field irrigation?
Multiply your crop's daily evapotranspiration (ETc in mm/day) by the field area in hectares, then divide by irrigation system efficiency to get the gross daily volume in m³. Multiply by 10 to convert to litres, then multiply by the number of days between refills and add a 10–20% safety buffer. This calculator does all of that automatically from your inputs.
What is ETc and where do I find it?
ETc stands for crop evapotranspiration — the combined water lost from the soil surface (evaporation) and from the crop itself (transpiration). FAO Irrigation and Drainage Paper 56 (FAO-56) is the global standard reference and provides Kc crop coefficients used to calculate ETc from local reference evapotranspiration (ETo). Your national or regional agricultural extension office will have local ETo data. For this calculator, the dropdown provides typical peak-season ETc values you can use as a starting point.
How much water does 1 hectare of crops need per day?
At a typical ETc of 5 mm/day and a 75% efficient sprinkler system, the gross application rate is 6.67 mm/day. Over 1 hectare, that equals 66.7 m³/day, or 66,700 litres per day. For drip irrigation at 90% efficiency, the same crop needs only 55,600 litres per day — 16% less water from the tank for the same field and crop.
Why is a safety buffer recommended when sizing an irrigation tank?
ETc values are averages — a hotter-than-normal week, a delivery delay, pump downtime, or minor leaks can push demand 10–20% above the calculated figure. Sizing to exactly the theoretical minimum means running dry under any deviation from perfect conditions. A 10–15% buffer costs little extra but prevents crop stress during the worst days of the growing season.
What is the difference between net and gross irrigation water requirement?
Net irrigation requirement is the water the crop actually needs (ETc minus effective rainfall). Gross requirement is what you must apply at the inlet, accounting for system losses. For a net demand of 5 mm/day with a 75% efficient sprinkler system, the gross requirement is 5 ÷ 0.75 = 6.67 mm/day. The extra 1.67 mm is lost to evaporation, runoff, and system inefficiency — you still have to source it and pump it, even though the crop never uses it.
Calculator for irrigation tank size: how large for a 5-hectare vegetable farm?
Using tomatoes at 4 mm ETc/day, drip irrigation (90% efficiency), 2-day refill interval, and a 10% buffer: Gross = 4 ÷ 0.9 = 4.44 mm/day. Daily volume = 4.44 × 5 ha × 10 = 222 m³/day = 222,000 L/day. Over 2 days with 10% buffer: 222,000 × 2 × 1.10 = 488,400 litres. You would typically select two 250,000-litre tanks or a single 500,000-litre underground cistern.
Should I account for rainfall when sizing an irrigation tank?
This calculator sizes for full irrigation without rainfall credit — the conservative and correct approach for peak-season design. If reliable in-season rainfall is predictable in your region, you can reduce the ETc value you enter by the average effective rainfall (in mm/day). However, for structural tank sizing, always design for the worst-case dry spell rather than the average year.
How to size a tank for drip irrigation vs sprinkler irrigation?
Use the same area, ETc, interval, and buffer but change the efficiency. Drip at 90% versus sprinkler at 75% means you need 83% as much tank capacity for drip — roughly 17% smaller. Over 5 hectares with a 3-day refill, this can be the difference between a 300,000-litre and 360,000-litre tank — a meaningful cost difference at procurement. Use this calculator to compare both scenarios side by side before committing to a system.