Instantly calculate the volume, weight and daily supply capacity of any storage tank. Supports cylindrical and rectangular shapes in metric or imperial units — no signup required.
Last reviewed: August 2026 · Formulas verified against standard engineering references
Whether you're designing a rainwater harvesting system or sizing a backup domestic water storage cistern, accuracy matters. This calculator uses standard geometric formulas — cylinder volume (πr²h) and rectangular volume (L×W×H) — to give you precise, engineering-grade estimates for both tank shapes.
Say you have a rectangular tank 6 ft long, 4 ft wide, and 5 ft deep. Volume = 6 × 4 × 5 = 120 cubic feet. Since 1 cubic foot ≈ 28.3 liters, that's roughly 3,398 liters (about 898 gallons) — weighing close to 3,398 kg (7,491 lbs). At 150 liters per person per day, that tank alone could supply 4 people for about 5.6 days.
The right shape depends on your available space and structural needs. Vertical cylinders distribute water pressure evenly around a curved wall, making them stronger per unit of material — the standard choice for large outdoor storage. Rectangular tanks maximize floor space in tight areas like basements or crawl spaces, but need thicker walls or internal bracing to resist the "corner stress" created by water pushing outward on flat panels.
| Common Use | Typical Capacity | Approx. Dimensions (Cylinder) |
|---|---|---|
| Small household backup | 500 - 1,000 L | ~1 m dia × 1.3 m ht |
| Family of 4, full supply | 2,000 - 3,000 L | ~1.5 m dia × 1.7 m ht |
| Rainwater harvesting (small home) | 5,000 L | ~2 m dia × 1.6 m ht |
| Farm / agricultural use | 10,000+ L | ~2.5 m dia × 2 m ht |
Water volume units vary by region. Most of the world measures potable water storage in liters or cubic meters; the United States uses gallons and cubic feet. One cubic foot of water holds approximately 7.48 gallons or 28.3 liters. This calculator shows all three measurements at once, so you can cross-reference pump specs and utility charts without manual conversion.
Water weight is one of the most overlooked factors in tank installation. One liter weighs exactly one kilogram (2.2 lbs), and one cubic meter weighs 1,000 kg (2,204 lbs). A 5,000-liter tank on a roof or elevated platform adds 5 metric tons of load. This calculator's weight output is essential for a structural engineer or contractor sizing the support foundation and pump head.
Once sized and installed, proper maintenance keeps stored water safe. Tanks should be opaque to block sunlight and prevent algae growth, and cleaned at least once every one to two years to remove sediment. For well water storage, make sure the tank is sealed against insects and small animals. Sizing correctly with this calculator also avoids "stagnant water" issues caused by an oversized tank with a slow turnover rate.
Q: How many gallons of water does a family of four use per day?
A: On average, a family of four uses between 200 and 400 gallons (roughly 750-1,500 liters) per day for showers, toilets, laundry and cooking. For emergency storage, plan for at least a 3-day supply per person.
Q: What is "pumping head" in a water system?
A: Pumping head is the vertical distance the water must be lifted plus the friction loss in the pipes. Your tank height and elevation help determine the right pump power.
Q: Are plastic (poly) tanks better than concrete?
A: Plastic tanks are lighter, easier to install, and generally cheaper. Concrete tanks are more durable, can be buried underground, and keep water cooler in hot climates.
Q: How do I calculate the capacity of a swimming pool?
A: Use the rectangular shape option and enter the average depth. For irregular pools, break the area into smaller rectangles and sum the results.
Q: What size water tank do I need for a family of 4?
A: For a 3-day emergency reserve at 150 liters per person per day, that's about 600-900 liters. For a full household backup supply, 2,000-5,000 liters is typical.
Q: How often should a water storage tank be cleaned?
A: Inspect and clean most residential tanks every 1-2 years to remove sediment and prevent algae or bacterial growth — more frequently in warm climates.
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