Enter your field area, crop type, irrigation method, and refill interval to calculate exactly how large your storage tank needs to be — no undersizing, no costly overflow.
Fill in your field and crop details — all outputs update instantly
1. Field & Crop Parameters
Field Area
Crop Type
Peak ETo
Reference ET for your hottest month (weather service or FAO CLIMWAT). Blank uses 6 mm/day, typical of a hot, dry summer.
Crop Water Use (ETc)
—mm/day
ETc = Kc × ETo (FAO-56). Peak-season Kc mid from FAO-56 Table 12.
Custom ETc (mm/day)
2. Irrigation System & Schedule
Method
Efficiency (%)
Refill Interval
Safety Buffer
3. Water Source optional
Source Yield
Pumping Hours / Day
hrs
Borehole, river pump or mains connection. Blank uses 20 hours (rest time for the pump). Ignored for m³/day.
⚠ Please enter a valid field area and refill interval (both must be greater than zero).
WaterTankCalculator.com — Field Irrigation Tank Sizing Results
Your Results
Recommended Tank Capacity
—
Litres (L)
Cubic Metres
— m³
US Gallons
— gal
UK/Imperial Gallons
— gal
Daily Crop Demand
— L/day
Gross Water Need
— mm/day
Water Weight (full)
— kg
Crop ETc (Net)
— mm/day
Source Supplies
— m³/day
Max Area Source Can Water
— ha
Tank Fill Indicator
📖 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:
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.
Select your crop type
Choose from the dropdown. Each option carries its peak-season crop coefficient (Kc mid) from FAO-56 Table 12. Then enter the reference ET (ETo) for your hottest month — the calculator works out crop water use as ETc = Kc × ETo. If you already have an ETc figure from your extension service, select "Custom ETc" and enter it directly in mm/day.
Pick your irrigation method and efficiency
Drip systems deliver water directly to the root zone and waste very little — FAO uses 90% as a typical field application efficiency for drip, 75% for sprinklers and 60% for surface irrigation. Select the method closest to yours; or choose "Custom efficiency" to enter a specific percentage.
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.
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.
Read your result
The primary figure is the recommended tank capacity in litres, including your buffer. The grid shows daily demand, net and gross application rates and weight. If you entered your water source, it also shows whether the source can keep up and the largest area it can irrigate. Use the copy button to save results for procurement.
Tip: This calculator sizes for peak-season demand. The crop list uses Kc mid, the peak-season value, and you should enter the ETo of your hottest month — together they give the worst-case tank requirement. For stage-by-stage figures, use the Crop Water Need Calculator.
📐 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. FAO-56 does not publish fixed ETc values, because crop water use depends on the weather; it publishes crop coefficients (Kc) that you multiply by the reference ET (ETo) for your location.
Step 1 — Net water need (ETc)
ETc (mm/day) = Kc × ETo (FAO-56)
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 Range
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
Ranges vary with design, maintenance and management. FAO's Irrigation Scheduling manual uses 90% (drip), 75% (sprinkler) and 60% (surface) as indicative field application efficiencies — the calculator defaults follow these. Use a measured figure if you have one.
Typical Crop Water Needs (ETc, Peak Season)
Crop
Kc mid
ETc at ETo 4
ETc at ETo 6
ETc at ETo 8
Wheat / Barley
1.15
4.6
6.9
9.2
Maize / Corn
1.20
4.8
7.2
9.6
Rice (paddy)
1.20
4.8
7.2
9.6
Tomatoes
1.15
4.6
6.9
9.2
Cotton
1.20
4.8
7.2
9.6
Sugarcane
1.25
5.0
7.5
10.0
Citrus
0.65
2.6
3.9
5.2
Alfalfa
1.20
4.8
7.2
9.6
Pasture / Grass
0.95
3.8
5.7
7.6
Potatoes
1.15
4.6
6.9
9.2
ETc in mm/day = Kc mid × ETo. Kc mid from FAO-56 Table 12 for a sub-humid climate (citrus: no ground cover, ~70% canopy; alfalfa: peak between cuttings; pasture: rotated grazing, 0.85–1.05). FAO-56 Table 2 puts typical ETo at about 2–4 mm/day in moderate humid climates and 6–9 mm/day in warm arid ones. Rice paddies also lose water to percolation and puddling, which is not included here.
Matching the Tank to Your Water Source
A tank only stores water — it does not create it. Over any run of hot days, your water source has to supply the full gross daily demand, or the tank drains a little more each day until it is empty. Before you buy a tank, compare the two:
Source supply (m³/day) = Yield (m³/hr) × Pumping hours per day
Max irrigable area (ha) = Source supply ÷ (Gross mm/day × 10)
If the source supplies less than the daily demand, a bigger tank will not fix it. Reduce the irrigated area, switch to a more efficient method, or find more water. The calculator runs this check when you enter your source yield.
🌾 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.
💧
Drip Irrigation
Use a higher efficiency (90%) and smaller tank — but never skip the safety buffer. Emitters clog, pressure drops, and flow rates vary.
🌀
Sprinkler Systems
Wind drift and evaporation during spraying are the biggest losses. Irrigating in the early morning or evening, when it is cooler and calmer, reduces both.
🏞️
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.
📅
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.
🧮 Worked Example
Worked Example: 3 Hectares of Drip Tomatoes on a Borehole
A grower plans 3 ha of drip-irrigated tomatoes. The hottest month has an ETo of 6 mm/day. The borehole yields 8 m³/hr and the pump can run 20 hours a day. The tank will be refilled daily with a 10% buffer.
Step
Working
Result
Crop water use
Kc 1.15 × ETo 6
6.9 mm/day
Gross application
6.9 ÷ 0.90
7.67 mm/day
Daily demand
7.67 × 3 ha × 10
230 m³/day
Tank
230 m³ × 1 day × 1.10
253 m³ (253,000 L)
Source supply
8 m³/hr × 20 hrs
160 m³/day ✗
Max area the source can water
160 ÷ (7.67 × 10)
2.09 ha
The tank size is right for 3 ha, but the borehole falls 70 m³ short every peak day — the tank would be empty by the second day of a heatwave. The real decision is not the tank: plant about 2 ha at peak, stagger planting so all 3 ha don't peak together, or add a second water source. At 2 ha, demand drops to 153 m³/day, just inside the 160 m³/day the borehole can deliver, and the tank comes out at about 170 m³. With only 7 m³/day to spare, though, a tank drawn down by a one-day pump failure would take about three weeks of peak-season pumping to recover. Planting 1.5–1.8 ha at peak (115–138 m³/day) leaves a more comfortable margin.
❓ FAQ
Frequently Asked Questions
How do I calculate the water tank size for field irrigation?
Work out crop water use as ETc = Kc × ETo (mm/day), then divide by irrigation efficiency to get the gross application in mm/day. Multiply by the field area in hectares and by 10 to get cubic metres per day (1 mm over 1 ha = 10 m³), and by 1,000 for litres. 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 weather service, state ET network or agricultural extension office will have local ETo data, and FAO's CLIMWAT database gives monthly ETo for thousands of stations. In this calculator, the crop dropdown supplies the FAO-56 Kc mid value and you enter your ETo; it then shows the resulting ETc.
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?
Take tomatoes at peak (Kc 1.15) in a hot summer with ETo of 6 mm/day: ETc = 6.9 mm/day. With drip at 90% efficiency, gross = 6.9 ÷ 0.9 = 7.67 mm/day. Daily volume = 7.67 × 5 ha × 10 = 383 m³/day. With a 2-day refill interval and a 10% buffer: 383 × 2 × 1.10 = 843 m³ (843,000 litres). At this size most farms use a lined reservoir or pond rather than tanks. In a milder climate with ETo of 4 mm/day the same field needs about 562 m³. Check that your water source can supply 383 m³ a day before you build.
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 choose "Custom ETc" and enter ETc minus the dependable 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.
✅ Sources & Review
Sources and review
The crop coefficients, efficiencies and conversions on this page come from these references:
The micro-spray and flood efficiencies in the method list and the 20-hour default pumping time are typical engineering values rather than figures from the sources above; replace them with your own measured numbers when you have them.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
