Irrigation & Water
Water Tank Calculator
Calculate water tank capacity in gallons from tank dimensions.
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How to Use This Calculator
Select the tank shape — round (cylindrical) or rectangular. For round tanks, enter the diameter and height. For rectangular tanks, enter the length, width, and height. All dimensions should be in feet. The calculator shows capacity in gallons and cubic feet.
Why This Matters
Knowing your exact tank capacity prevents overflows, ensures adequate water storage for irrigation cycles, and helps you calculate how long stored water will last at your planned application rate. Proper sizing also avoids wasting money on oversized tanks.
Methodology
Round tank volume = π × (Diameter/2)² × Height in cubic feet. Rectangular tank volume = Length × Width × Height. Gallons = Cubic feet × 7.481. One cubic foot holds 7.481 gallons. A 10-ft diameter, 8-ft tall round tank holds about 4,696 gallons.
Common Mistakes to Avoid
- Confusing radius and diameter — make sure you enter the full diameter, not the radius.
- Not allowing for 10-15% headspace to prevent overflow.
- Forgetting that elevated tanks lose capacity to the stand height but gain pressure.
- Ignoring the weight of water — water weighs 8.34 lbs/gallon, so a 5,000-gallon tank weighs over 20 tons when full.
Tips & Best Practices
- Allow 10-15% extra capacity for expansion and overflow.
- Elevated tanks provide gravity-fed pressure for irrigation.