How Much Does a Gallon of Water Weigh? (Exact Numbers by Temperature and Salinity)

A gallon of water weighs 8.34 lb at 62°F — but that single number hides three variables most charts ignore: temperature shifts it by up to 4%, dissolved minerals by almost nothing, and the imperial gallon is a different unit entirely (10 lb by its original legal definition). This page gives you the full temperature table, the fresh/salt/hard water comparison, and one plumbing-and-filters angle you will not find on the ranking calculator sites: what those pounds mean flowing through a fridge filter every six months. Related: How Many Ounces in a Gallon and What Is TDS in Water?

Quick Answer: A US liquid gallon of water weighs 8.34 lb (3.78 kg) at 62°F (17°C). Cold water is marginally denser (8.345 lb at 39.2°F, water’s maximum density); near-boiling water is lighter (8.00 lb at 212°F). Dissolved minerals barely register: even very hard tap water (250 mg/L TDS) adds only ~0.004 lb per gallon.

The top Reddit result for this question is an ELI5 thread titled “Why is a gallon of water equal to approximately 8.34 pounds” — and the replies split into two camps: people who multiply 3.785 liters by 2.2 lb, and aquarium owners who point out their 5-gallon tank refill “feels way heavier after adding marine salt mix.” Both camps are correct for their own water. The number only becomes exact once you pin the temperature and what’s dissolved in it.

Why Water Gets Lighter as It Heats (The Full Table)

Water density peaks at 39.2°F (4°C) at 999.97 kg/m³ and falls off in both directions from there — above freezing because molecular motion expands the structure, below 4°C because ice-like domains begin forming toward freezing. Every weight figure on this page is derived from density values published by NIST (National Institute of Standards and Technology) and the International Association for the Properties of Water and Steam.

The practical spread: from ice-cold to boiling, a gallon loses about 0.34 lb — roughly 4%. For household purposes (weighing a full aquarium, sizing a shelf for water jugs, calculating a water bed) the 62°F value of 8.34 lb is the engineering convention, and it is the constant the EPA uses in drinking-water treatment calculations.

One number worth committing to memory: a cubic foot of water weighs 62.4 lb (at 4°C). A 40-gallon water heater holds about 334 lb of water before you count the tank — which is why tank failures flood basements.

Fresh vs Salt vs Hard Water: Weighing What’s Dissolved

Seawater is the outlier. At 35 parts per thousand dissolved salt, a gallon weighs 8.56 lb — about 2.6% more than fresh. Each 1 ppt of salinity adds roughly 0.13% to the weight. This is why marine aquarium owners lift noticeably heavier buckets than freshwater owners at the same volume.

Hard water barely registers — and this is the part most filter-site articles get wrong. Water at 250 mg/L TDS (officially “very hard” by USGS classification) carries 250 grams of dissolved minerals per thousand liters, which works out to about 0.004 lb per gallon. You cannot feel it, and your scale cannot reliably measure it. Hard water’s real signature is on your fixtures and heating elements, not on a bathroom scale. What TDS actually measures and why it matters for filter choice is covered in our TDS guide.

The Imperial Gallon: 10 Pounds, By 1824 Law

Here is the accounting exercise no US ranking page runs: the imperial gallon was legally defined as the volume of 10 lb of distilled water at 62°F — Britain’s Weights and Measures Act of 1824 anchored the unit to weight itself. That is why an imperial gallon (4.546 L) holds 160 imperial fluid ounces versus the US gallon’s 128 US fluid ounces, and why a UK gallon of water weighs 10.02 lb, not 8.34.

The US gallon, by contrast, descends from the old English wine gallon — 231 cubic inches, a volume standard from 1706 — which the US kept when Britain reformed. So when a British recipe or a Canadian car manual says “gallon,” it is describing 20% more water than a US source. Cross any border with a fuel or water calculation and this 1.2× factor is where the error hides.

What Flows Through Your Fridge Filter in Six Months

The scale exercise that makes this concrete: a fridge filter certified for 200 gallons per cycle, replaced on the common six-month schedule, has 1,668 lb of water pass through its carbon media — three-quarters of a ton through a cartridge the size of a soup can. A family of four running through the upper range of 500 gallons pushes 4,170 lb through in the same period.

Every one of those gallons carried whatever your supply line delivered: chlorine, chloramine, sediment, and in older neighborhoods, trace lead loosened by the pipes upstream. That is the entire logic of filter replacement scheduling — the media exhausts by gallons processed, not by calendar. If your household drinks two gallons a day, a 200-gallon cartridge is fully consumed in about three months, no matter what the sticker on the door says. More on that: How Often to Change a Refrigerator Water Filter.

FAQ

What is heavier, a gallon of milk or a gallon of water?

Milk. A US gallon of whole milk weighs about 8.6 lb versus 8.34 lb for water — milk is roughly 87% water plus 3.25% fat, proteins, and lactose (milk solids are denser than water, and the fat is lighter, netting out to about 3% heavier).

How much does a 5 gallon jug of water weigh?

41.7 lb for pure water at 62°F (5 × 8.34). Add the jug itself (typically 1–2 lb for HDPE) and office water coolers are being lifted at 43–44 lb — which is exactly why 5-gallon jugs have ergonomic handles and why lifting technique matters.

How much weight is 1 gallon of milk?

About 8.6 lb (3.90 kg) for whole milk; skim milk runs slightly denser at ~8.63 lb per gallon since removing fat leaves a higher proportion of water and dissolved solids.

What is heavier, a gallon of water or a gallon of ice?

A gallon of water is heavier. Ice is about 9% less dense (0.917 g/cm³ vs 0.9998 g/cm³), so a gallon-volume of ice weighs roughly 7.65 lb — and it floats, with about 92% of an iceberg submerged.

Sources & References

• NIST — thermophysical properties of water (density vs temperature), used for all weight derivations. Official documentation.
• UK Weights and Measures Act 1824 — imperial gallon defined as 10 lb distilled water at 62°F. Official documentation (historical regulation).
• USGS — water hardness classification (0-60 soft / 61-120 moderately hard / 121-180 hard / >180 mg/L very hard). Official documentation.
• EPA — 8.34 lb/gal as the standard constant in drinking water treatment calculations. Official documentation.
• Reddit r/explainlikeimfive thread — SERP #1 for this query; user intuitions referenced (not quoted). Community discussion.

Related Guides

• How Many Ounces in a Gallon? — 128, 160, or 148.95: which gallon you’re converting matters.
• What Is TDS in Water? — what those dissolved solids are and when they matter.
• How Often to Change a Refrigerator Water Filter — the gallons-processed logic behind the six-month rule.