1 L of water ≈ 1 kg ≈ 10 N of weight. A cubic metre is a tonne.
Water’s density of 1,000 kg/m³ makes it the universal mental anchor for mass, force and volume checks.
At 4 °C and fresh. Seawater is about 1.025 kg/L, and water near boiling falls to 0.958 — so a 1,000 L tank of hot water weighs about 42 kg less than the cold one, which matters if you sized the bracket from the cold figure.
At 20 °C the rule says 1 kg and the exact answer is 0.9982 kg — within half a percent. It holds to within 2% up to 65.2 °C, then drifts.
True to within a gram in a kitchen and out by 4% in a kettle - water is densest at 4 °C and loses body as it heats.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
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