How much does water weigh?
One US gallon of water weighs 8.34 lb (3.79 kg). One cubic foot of water weighs 62.4 lb (28.3 kg). One cubic yard weighs 1,686 lb (765 kg) — nearly a short ton.
Skip the arithmetic: use the water weight calculator → Enter any volume — gallons, litres, cubic feet or cubic metres — and get the weight instantly.
1. The short answer — water density and what it means
Water density is the mass of water per unit volume. The standard engineering value is 1,000 kg/m³, which equals 1 kg per litre or 62.4 lb per cubic foot. This is the figure used by structural engineers, ASHRAE, and most plumbing and building codes when calculating water loads on structures.
In practice, water density varies slightly with temperature. Pure water is densest at 3.98 °C (999.97 kg/m³) and becomes slightly less dense as it warms — at 20 °C it is 998.2 kg/m³, and at 60 °C it drops to 983 kg/m³. For all construction, plumbing and civil engineering purposes the difference is less than 2%, so 1,000 kg/m³ (1 kg/L) is used universally and is what this calculator applies.
The litre was originally defined as the volume of exactly one kilogram of water at its maximum density (4 °C), which is why the relationship 1 L = 1 kg is so clean. Modern SI redefined the litre in terms of cubic metres (1 L = 0.001 m³) without changing the practical result.
2. Water weight reference table
The table below covers the volumes most commonly needed on a jobsite or in a structural check. All weights use the standard engineering density of 1,000 kg/m³.
| Volume | US gallons | Litres | kg | lb |
|---|---|---|---|---|
| 1 US gallon | 1 | 3.79 | 3.79 | 8.34 |
| 1 UK gallon | 1.20 | 4.55 | 4.55 | 10.02 |
| 5 US gallons | 5 | 18.93 | 18.93 | 41.7 |
| 1 cubic foot | 7.48 | 28.32 | 28.32 | 62.4 |
| 50 US gallons | 50 | 189.3 | 189 | 417 |
| 100 US gallons | 100 | 378.5 | 379 | 834 |
| 100 litres | 26.4 | 100 | 100 | 220 |
| 500 US gallons | 500 | 1,893 | 1,893 | 4,170 |
| 1 cubic yard | 202 | 764.6 | 765 | 1,686 |
| 1,000 litres (1 m³) | 264 | 1,000 | 1,000 | 2,205 |
| 1,000 US gallons | 1,000 | 3,785 | 3,785 | 8,345 |
Weights rounded to 3–4 significant figures. Density = 1,000 kg/m³ (standard engineering value).
3. Why water weight matters in construction
Water is heavier than most people expect. A single cubic foot — a box about the size of a standard milk crate — holds 62.4 lb of water. That weight adds up quickly in construction:
- Water storage tanks and cisterns. A 500-gallon above-ground tank holds nearly 4,200 lb (1,900 kg) of water when full. The floor, slab or structural frame below must be designed to carry this live load. Residential floors are typically designed for 40 psf — a full 500-gallon tank concentrated on a small footprint often exceeds this and requires point-load engineering.
- Swimming pools. A modest 12 × 24 ft pool (4 ft average depth) holds about 8,600 US gallons — approximately 71,700 lb or 36 short tons of water. Even small above-ground pools require level, load-rated ground preparation.
- Rooftop ponding. Water ponding on a flat roof after heavy rain is a serious structural concern. Just 1 inch of standing water over a 1,000 sq ft roof adds about 5,200 lb (2,360 kg) of unplanned load. Roofs are designed for specified ponding loads; trapped water can trigger progressive collapse if drainage fails.
- Pipe systems.Water-filled pipes add significant weight, especially in large-bore systems. A 100 mm (4-inch) pipe 30 m long holds about 235 kg of water when full — on top of the pipe's own weight. Hangers and supports must account for full-pipe weight, not just the pipe itself.
- Rainwater harvesting and IBC totes. A standard 1,000-litre IBC tote weighs about 1,000 kg (2,205 lb) when full. Two totes on a mezzanine floor add over 4,400 lb — well beyond a standard 250 psf warehouse floor rating if the load is concentrated on a small area.
4. How to calculate water weight by hand
The formula is straightforward: multiply the volume by the density of water.
Weight (kg) = Volume (litres) × 1
Weight (lb) = Volume (US gallons) × 8.34
Weight (lb) = Volume (ft³) × 62.4
Worked example 1 — water heater tank. A 270-litre (71-US-gallon) water heater tank holds 270 kg (595 lb) of water when full. Add the empty tank weight (roughly 50–80 kg for a steel tank) and the total installed weight exceeds 320–350 kg. This is why manufacturers specify floor reinforcement for large water heaters and why the water heater size calculator is a useful companion tool.
Worked example 2 — rainwater harvesting tank.You want to install a 1,000-gallon (3,785 L) polyethylene tank on a concrete pad. The water alone weighs 3,785 kg (8,345 lb) — about 3.8 metric tonnes or 4.2 short tons. The concrete pad must be designed for at least this load plus the tank's own weight, the weight of any connecting pipes, and a safety factor.
Worked example 3 — above-ground pool. A round 14 ft (4.27 m) diameter pool filled to 4 ft (1.22 m) holds about 4,600 US gallons (17,400 L) of water — approximately 38,400 lb (17,400 kg). The deck boards, footings and ground preparation must all be rated for this load. Use the pool volume calculator to find the volume first, then multiply by 8.34 lb/gal (or 1 kg/L) to get the water weight.
5. US gallon vs UK gallon — why the difference matters
The US gallon and the UK (imperial) gallon are different units. This causes confusion when reading international specifications, tank datasheets or pump curves:
| Unit | Litres (exact) | kg of water | lb of water |
|---|---|---|---|
| 1 US gallon | 3.785411784 | 3.785 | 8.34 |
| 1 UK gallon | 4.54609 | 4.546 | 10.02 |
The UK gallon is about 20% larger than the US gallon. If a pump datasheet quotes flow in “gallons per minute” without specifying US or UK, the difference matters — a 10 gpm US pump moves 37.9 L/min; a 10 gpm UK pump moves 45.5 L/min. Always check the specification sheet. The GPM flow rate calculator handles both unit systems.
6. Frequently asked questions
- How much does a gallon of water weigh?
- One US gallon of water weighs 8.34 lb (3.79 kg). One UK (imperial) gallon is larger — 4.546 litres — so it weighs 10.02 lb (4.55 kg). The standard engineering density is 1,000 kg/m³ (1 kg per litre).
- How much does a litre of water weigh?
- One litre of water weighs 1 kg (2.20 lb). This comes directly from the definition of the litre — historically defined as the volume of one kilogram of water at 4 °C. At typical working temperatures (10–25 °C) the density is 997–1000 kg/m³, so 1 kg/L is accurate enough for all construction and engineering calculations.
- How much does a cubic foot of water weigh?
- One cubic foot of water weighs 62.4 lb (28.3 kg). Since 1 ft³ = 28.317 litres and 1 litre = 1 kg, the mass is 28.3 kg, or 28.3 × 2.205 = 62.4 lb. Structural engineers commonly use 62.4 pcf as the unit weight of water for design loads.
- How much does 5 gallons of water weigh?
- Five US gallons of water weigh 41.7 lb (18.9 kg). A standard 5-gallon bucket is a common jobsite carry — knowing its weight matters for manual handling ergonomics and falls within the typical one-person lift limit. A UK 5-gallon equivalent (22.7 L) weighs 22.7 kg / 50.1 lb.
- How much does a cubic metre of water weigh?
- One cubic metre of water weighs exactly 1,000 kg (1 metric tonne, or 2,205 lb). This is the most useful figure for structural engineers checking tank and pool loads — 1 m³ = 1 tonne of water.
For pipeline and pressure system work, the pipe volume calculator gives the volume of water in a pipe run, and the pipe weight calculator adds the weight of the pipe itself. For pool water volume, use the pool volume calculator.
Last reviewed 2026-09-02.