How this instrument works
The US gallon is a customary volume unit fixed at exactly 3,785.411784 millilitres — a number set by federal weights-and-measures definitions rather than derived from any particular container. Fuel tanks, paint cans, and chemical drums are all commonly sized and sold in gallons, yet none of those industries can tell you a container's weight from its gallon rating alone: a 5-gallon can of gasoline, a 5-gallon can of paint, and a 5-gallon jug of water share the exact same size and land on three very different numbers on a scale.
That's the gap the Density field closes, and its answer changes with what's actually inside the container. Gasoline typically sits near 0.74 g/mL, diesel around 0.85, common paint formulations anywhere from about 1.2 to 1.4 depending on the pigment load, and liquid propane closer to 0.51 — a wide spread that no fixed factor could cover honestly. Water's 1.00 is loaded first simply because it's the most familiar reference point, not because it's the right answer for a fuel tank or a paint can.
- Enter your volume into Volume (US gallons) — 5 is preloaded, a common jug or small drum size.
- Set Density (g/mL) to your liquid; 1.00 (water) is only a starting point, change it for fuel, paint, or anything else.
- Mass (kg) recalculates immediately below both fields.
- Weighing a full tank for towing or shipping? Add the container's own empty weight separately — this only converts the liquid itself.
Worked example — 5 gallons of water, then gasoline
Five US gallons at the default Density of 1.00 gives Mass (kg) = 18.92705892 — consistent with the familiar figure that a single gallon of water weighs close to 3.785 kg, or about 8.34 lb.
Refill that same 5-gallon container with a lighter liquid around 0.80 g/mL — kerosene or a similar light fuel sits in that range — and Mass (kg) drops to 15.141647136, nearly 3.8 kg less for the same 5 gallons. Ordinary gasoline, closer to 0.74 g/mL, would weigh even less again; the container never changed size, only what was poured into it.
Questions
Why doesn't 1 gallon just equal one fixed number of kilograms?
Because mass depends on the liquid's density, and a gallon only measures volume. Only water sits near the commonly quoted 8.34 lb (about 3.785 kg) per gallon, and even that shifts a little with temperature; fuels, paints, and most other liquids land on their own, different weight for the same gallon.
How do I find the density of my specific fuel or liquid?
Check the product's safety data sheet or manufacturer spec, which almost always lists density or specific gravity directly. For common liquids without documentation handy, a published reference table gives a reasonable starting estimate, though the exact figure can vary between brands and formulations.
Does temperature affect the answer?
Yes, especially for fuels and other volatile liquids, where density drops noticeably as temperature rises. If precision matters — for billing or a tight weight limit — use a density value measured at or close to the liquid's actual temperature rather than a generic room-temperature figure.
What's a rough density for gasoline, diesel, or paint?
Gasoline typically runs about 0.72–0.75 g/mL, diesel around 0.83–0.87 g/mL, and paint commonly 1.2–1.4 g/mL depending on its formulation. Treat these as reasonable starting points rather than guarantees for any specific product you actually have on hand.
Is a US gallon the same as an imperial (UK) gallon?
No — the US gallon (about 3.785 L) is noticeably smaller than the imperial gallon (about 4.546 L) used in the UK and some other countries. This calculator is built around the US gallon specifically, so a UK-labeled container needs its own conversion first before these numbers apply.
Can I use this to estimate how much a full tank adds to a vehicle's weight?
Yes, for the liquid itself, once you have a reasonable density for that fuel. Remember to add the tank and container's own material weight separately, and keep in mind that fuel gauges rarely reflect a truly full tank, so treat the volume you enter as an estimate too.