Water Tank Size Calculator
Estimate household or site water-storage capacity from users, daily consumption, autonomy days, reserve and usable tank percentage.
Default project preview
733 gallons
Full tanks
2
Purchased capacity
1,000 gal
Usable stored water
900 gal
Supply duration
4.5 days
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Water users
Enter this project assumption in a unitless value. The current imperial default is 4. Minimum: 1.
Daily use per person
Enter this project assumption in gal/day. The current imperial default is 50 gal/day.
Storage autonomy
Enter this project assumption in days. The current imperial default is 3 days.
Demand reserve
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Usable tank capacity
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 10. Maximum: 100.
Capacity per tank
Enter this project assumption in gal. The current imperial default is 500 gal.
Price per tank
Enter this project assumption in a unitless value. The current imperial default is 850. Minimum: 0.
How to read the outputs
Full tanks
500 gal each. Default preview: 2.
Purchased capacity
full tank modules. Default preview: 1,000 gal.
Usable stored water
90% usable. Default preview: 900 gal.
Supply duration
at entered daily use. Default preview: 4.5 days.
Full water weight
water only. Default preview: 8,345 lb.
Tank cost
tanks only. Default preview: $1,700.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Water demand = users × daily use × autonomy days. Reserve is added, then divided by usable tank percentage. Tank count rounds up to full modules.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Estimate preliminary storage volume from daily demand, autonomy period and usable-storage allowance.
Calculation steps
- 1Enter the actual project dimensions or operating values.
- 2Apply the calculator formula using consistent units.
- 3Review the unrounded result and the editable allowance.
- 4Round only to the product, stock length or equipment increment actually purchased.
Worked numerical example
A property using 450 L/day with three days of autonomy and 10% unusable reserve needs about 1,500 L of nominal storage.
Purchasing and rounding
Choose the next standard tank size above the required usable storage after checking the manufacturer's usable volume, outlet elevation and any reserved dead volume. Also verify footprint, access, foundation loading and overflow routing before selecting a larger nominal tank.
Interpret the result
The calculated storage is the volume needed for the demand and autonomy assumptions entered. Nominal tank capacity can exceed usable drawdown, and the required size changes directly with daily consumption, refill reliability and the reserve you choose to maintain.
Page-specific assumptions
- Inputs describe one consistent project condition.
- Entered yields and efficiencies match the selected product or equipment.
- Site-specific code and engineering requirements are checked separately.
Common mistakes
- Mixing imperial and metric inputs.
- Using nominal dimensions where actual coverage controls.
- Rounding intermediate values too early.
- Treating a quantity estimate as design approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Water Tank Size Calculator questions
Why is usable capacity less than tank capacity?+
Outlet elevation, sediment space, overflow level and operating limits can prevent use of the full nominal volume.
How heavy is a full water tank?+
One litre of water weighs about one kilogram, and one US gallon weighs about 8.34 pounds, before tank weight.
Does this size the pump?+
No. Pump selection depends on flow, pressure, elevation, pipe loss and duty cycle.