Water Tank Capacity Calculator — Round, Square & Rectangular

Why Water Tank Capacity Matters — From Well Pressure Tanks to Rainwater Cisterns

Water tanks come in dozens of shapes and serve every need — a 200-liter polyethylene tank under a kitchen sink, a 1,000-liter well pressure tank in a rural basement, a 2,500-liter rainwater tank beside a suburban house, or a 50,000-liter fire suppression cistern at a commercial warehouse. Getting the volume right is not just an academic exercise. An undersized well tank means your pump cycles on and off every minute, wearing out the pressure switch in 18 months instead of 10 years. An oversized rainwater tank wastes yard space and costs twice as much as it needs to. And guessing the full weight of a tank before pouring a slab is how you end up with a cracked patio six months after install. This water tank capacity calculator gives you the exact liters, gallons, full weight and surface area for round, square and rectangular tanks in one click — so you can spec the right tank, the right slab, and the right pump the first time.

The rest of this page walks you through the formulas with worked examples you can follow on a scrap of paper, explains how to convert between liters and gallons without looking it up every time, gives you a standard sizes chart for common household and commercial tanks, shows a water weight table so you can estimate slab loading in your head, and covers the big installation decisions: above-ground versus underground, well tank sizing rules, and how to read a manufacturer's spec sheet without getting caught by nominal versus actual dimensions.

Round Water Tank Formula: V = π r² h — Explained with Worked Examples

The round (or cylindrical) water tank is by far the most common shape for household storage. Every well pressure tank, every standard rainwater tank, and most elevated water towers are vertical cylinders. The formula for the volume of a cylinder is one of the most basic in geometry — the area of the circular base multiplied by the height. Written out:

V = π × r² × h

Where V is volume, r is the inside radius of the tank (half the inside diameter), and h is the inside height. Use inside dimensions — not the outside numbers printed on the shipping box. A typical steel tank has walls 4–6 mm thick, which subtracts 8–12 mm from the inside diameter and another 4–6 mm from the inside height. Polyethylene tanks are thinner at 2–3 mm but still not zero. If you only have outside dimensions, subtract twice the wall thickness from the diameter and once from the height before applying the formula.

The easiest units to work in are meters. Volume in cubic meters converts directly to liters because 1 m³ = 1,000 L. If you are working in feet, 1 cubic foot equals 7.48 US gallons, and 1 cubic inch equals 1 / 231 US gallons. The calculator above handles all the unit conversion for you — you can enter meters or inches and get both liters and gallons back.

Worked Example 1 — 1.8 m × 2.0 m Round Tank

Worked Example 2 — 3.0 m × 4.0 m Round Tank

Worked Example 3 — 2.4 m × 3.0 m Round Tank

Rectangular Cistern: V = l × w × h

Rectangular water tanks are most commonly built on-site — concrete block cisterns, underground tanks excavated into a basement corner, or steel-panel tanks bolted together in a shipping yard. The formula is even simpler than the cylinder: multiply the three inside dimensions. No π, no square roots, no geometry homework.

V = length × width × height

Because rectangular tanks are almost always custom-built, the wall thickness matters more than it does for a factory-made PE tank. A typical concrete block cistern uses 150 mm (6 inch) hollow blocks filled with rebar and grout. That means the inside dimensions are 300 mm shorter in each horizontal direction than the outside footprint, and 150 mm shorter in height. Measure the inside of the blocks before pouring the slab — not after the walls go up.

Square Tank — Same Math as Rectangular

A square water tank is just a rectangular tank where the length equals the width. If the side is s and the height is h, the formula collapses to V = s² × h. Square tanks are less common than round or rectangular ones because a round tank gives more volume per surface area of wall material — but square tanks fit neatly in corners, stack more easily in a warehouse, and are sometimes built above ground with wooden or composite panels for rainwater harvesting on a budget.

Surface Area — Why It Matters for Slab and Base Design

The calculator on this page also returns the total surface area of the tank. You might be tempted to ignore that number — after all, you came for liters and gallons — but surface area is the key input for two critical design decisions: how much concrete you need for a supporting slab, and how much paint or liner you need if you are refurbishing an old steel tank.

For a round tank, surface area is the sum of the circular top, the circular bottom, and the cylindrical side wall: A = 2πr² + 2πrh. For a square tank it is the two square ends plus the four rectangular sides: A = 2s² + 4sh. For a rectangular tank it is all six faces: A = 2(lw + lh + wh). The calculator on this page gives surface area in square centimeters and square meters — use the square meter number for paint coverage (most tank epoxies cover 4–6 m² per liter) and for slab footprint sizing.

Water Weight When Full — 1 L = 1 kg, 1 US gal ≈ 8.34 lb

The most useful mental shortcut in water tank engineering: one liter of water weighs exactly one kilogram at standard temperature and pressure. That is not an approximation — it is how the kilogram was originally defined in the late 1700s (the mass of one liter of water at its maximum density). US customary units give you 8.33 pounds per gallon, rounded up to 8.34 for quick math. If you remember those two numbers, you can estimate the full weight of any tank in your head: multiply liters by 1 to get kilograms, multiply gallons by 8.34 to get pounds.

Water weight is always the dominant component of total system weight. The 5000 L tank you are considering puts 5 tonnes (5,000 kg) of water on the ground — plus another 100–200 kg for the polyethylene shell, or 800–1,200 kg for steel. Before you order a tank, grab your calculator and run: (liters × 1.0) + (shell weight estimate) × 1.5 safety factor = minimum slab load rating.

Water Weight Conversion Table — Liters ↔ Gallons ↔ kg ↔ lb

Table values use exact conversions: 1 L = 1 kg, 1 US gal = 3.78541 L, 1 kg = 2.20462 lb.

Standard Water Tank Sizes Chart — 200 L to 10,000 L

Water tanks follow surprisingly consistent nominal sizes across manufacturers and countries. Whether you are shopping for a replacement well tank in the US, a rainwater tank in Australia, or a backup cistern in Europe, the same round numbers — 200, 500, 1000, 2500, 5000 and 10000 liters — dominate catalogs. The chart below gives typical inside dimensions for standard round polyethylene tanks so you can cross-check against the capacity you calculate with this tool. Note that "nominal size" on a spec sheet is always the geometric capacity at full to the brim — usable capacity in a well pressure tank is about 60–70% of that number because of the trapped air cushion.

Above-Ground vs Underground Water Tanks — What the Formula Does Not Tell You

The geometry of a water tank is the same whether you bury it or set it on a slab — the volume formula does not care about elevation. But the installation math absolutely does. An above-ground tank needs a solid slab, UV protection if it is polyethylene, and insulation if winters get below zero. An underground tank needs excavation, a pump to move water back up to the house, a concrete base to prevent floating during heavy rain, and a leak detection system because a buried tank springing a leak is invisible until the lawn above turns into a swamp.

Frequently Asked Questions — Water Tank Capacity