How Much Does a Liter of Water Weigh?

How much does a liter of water weigh? Exactly 1 kilogram — about 2.2 pounds — at 4°C, the temperature of maximum density. At ordinary room temperature it is a hair lighter, and saltwater or syrup weighs more per liter. For quick weight-and-volume math, our free unit converter handles liters, gallons, ounces, and kilograms in one click.

The One-Kilogram Rule: Why a Liter of Water Weighs 1 kg

The answer is no accident: the kilogram was literally defined by water. In 1799, the French Academy defined the kilogram as the mass of one cubic decimeter of water at the temperature of melting ice — and a cubic decimeter is exactly one liter. So a liter of water weighs 1 kilogram by definition, not by coincidence. The modern kilogram has since been redefined in terms of the Planck constant (2019), but water's density was built into the original standard, which is why the relationship survives. This is also why metric volume and mass convert so cleanly: 1 milliliter of water weighs 1 gram, 1 liter weighs 1 kilogram, and 1 cubic meter weighs 1 metric ton. No other liquid gets this one-to-one deal — it is a historical artifact of water being the reference substance. Pure water at 4°C has a density of 0.99997 grams per milliliter, so a liter masses 999.97 grams — call it 1 kilogram and you are off by three-hundredths of a gram, an error your kitchen scale cannot detect.

How Much Does a Liter of Water Weigh in Pounds and Ounces?

In US customary units the numbers are uglier, because pounds and gallons were never defined by water. One kilogram equals 2.20462 pounds, so a liter of water weighs 2.20462 lb. Multiply by 16 and you get 35.274 ounces. The handy anchors worth memorizing: a 500 mL water bottle — the standard half-liter — holds about 1.1 pounds of water (plus roughly 10–15 grams for the plastic bottle itself). A 2-liter soda bottle of water weighs 2 kg of water, or about 4.4 pounds before the bottle. And the classic cook's shortcut — a pint's a pound the world around — is close but not exact: a US pint is 473 mL, so a pint of water weighs about 1.04 pounds. Running the reverse trip often? Our fluid ounces to liters converter turns bottle sizes into metric weight instantly.

Temperature Changes the Weight (Slightly)

Here is where the purist's answer diverges from the practical one. Water is densest at 3.98°C (about 39°F), where a liter masses essentially 1.000 kg. Warm it up and it expands — at a typical room temperature of 20°C (68°F), a liter of water weighs about 0.9982 kg, roughly 0.2% lighter. At a rolling boil (100°C), density drops to 0.9584 g/mL, so a liter weighs just 0.958 kg — about 4% lighter than the textbook value. Freeze it and water does something almost no other liquid does: it expands, which is why ice floats. A liter of ice weighs only about 0.917 kg while occupying the same volume as 1 kg of liquid water. For drinking, cooking, and carrying bottles, none of this matters — your recipe cannot tell 0.998 kg from 1.000 kg. It starts to matter in laboratories, in chemical dosing, and when you are shipping or storing thousands of liters, where a 0.2–4% density error becomes kilograms or tons.

How Much Does a Gallon of Water Weigh?

The gallon is where Americans most often meet water weight. A US gallon holds 3.785411784 liters, so a gallon of water weighs 3.785 kg × 2.20462 lb/kg = 8.345 pounds — commonly quoted as 8.34 lb/gal. That figure deserves a place in memory because it shows up everywhere: plumbing, aquariums, water heaters, and the RV world. The British imperial gallon is a different animal: at 4.54609 liters it holds 10.02 pounds of water — a full 20% more, which is exactly why the old saying 'a pint's a pound' holds better in the UK (their pint is 568 mL, weighing 1.25 lb — still not exact, but closer). Two facts worth pairing: a cubic foot of water is 7.48 gallons, so it weighs about 62.4 lb, and a cubic meter of water is 1,000 liters weighing 1 metric ton (2,204.6 lb). Our liters to gallons converter runs either direction in one click.

What About Saltwater, Milk, and Other Liquids?

The 1 kg/L rule only covers pure water. Dissolved or suspended stuff changes the density. Seawater averages about 1.025 kg/L (35 grams of salt per liter), so a liter of ocean water weighs roughly 2.26 pounds — this is why you float a touch easier in the sea. Milk runs about 1.028–1.035 kg/L depending on fat content; a gallon of milk weighs about 8.6 pounds, noticeably more than a gallon of water — our gallon of milk in liters guide works the full conversion. Vegetable oil is lighter than water at about 0.92 kg/L, which is why oil floats on water. Honey is dense at roughly 1.4 kg/L. And mercury sits at an absurd 13.6 kg/L — a single liter weighs nearly 30 pounds. The pattern: anything dissolved or suspended in water raises the weight per liter; anything oily or fatty lowers it.

Real-World Water Weight: Bottles, Jugs, and Cases

Theory is nice; here is what water actually weighs in the containers you carry. A standard 16.9 fl oz (500 mL) bottle holds 500 g of water plus ~10–15 g of PET plastic — about 1.13 pounds total. A 1-gallon jug is 8.34 lb of water plus ~0.15 lb of plastic, so figure 8.5 pounds in your hand. A 5-gallon water-cooler jug holds 41.7 lb of water — the reason delivery drivers use dollies. A 24-pack of half-liter bottles works out to about 26.5 pounds, the number our 24-pack of water bottles guide breaks down bottle by bottle. And a 55-gallon drum of water is a two-person (or forklift) job at roughly 500 pounds all-in — see our 55 gallon drum in liters guide for the full weight math. Why does any of this matter? Airline checked-bag limits (23 kg / 50 lb) are the classic trap: a checked bag stuffed with bottled water crosses the limit fast. Shipping water is famously uneconomical for the same reason — you are mostly paying to move weight.

Quick Answers

How much does a liter of water weigh in kg? 1 kilogram at 4°C; about 0.998 kg at room temperature. How much does a liter of water weigh in pounds? 2.20462 lb. Does a liter of hot water weigh less than cold? Slightly — a liter at boiling weighs about 0.958 kg versus 1.000 kg at 4°C. Does a liter of ice weigh 1 kg? No — freezing expands water, so a liter of ice weighs about 0.917 kg; the mass is unchanged, only the volume changes. How much does a liter of saltwater weigh? About 1.025 kg (2.26 lb). How much does a gallon of water weigh? 8.345 lb (US gallon) or 10.02 lb (imperial gallon). Keep the free unit converter and the ToolNest homepage bookmarked for every weight-and-volume conversion beyond water.