Endurance is subtraction, not measurement.
The ledger has four lines. Start with usable fuel — not tank capacity; every tank holds litres the pickup cannot reach. Subtract the taxi, run-up and climb allowance, which burns at a rate the cruise flow never sees. Subtract the reserve you have promised yourself not to touch. Divide what remains by an honest cruise flow. The result is the only number that matters, and it is in minutes.
Every term deserves suspicion in a different direction: usable fuel is a document number, the climb allowance is always underestimated, and the cruise flow is whatever your leaning discipline actually delivers — not what the book promised at a power setting you are not flying.
The lie of the top half.
Tank shapes make float gauges nonlinear: the gauge dwells contentedly on FULL while the widest part of the tank drains, then accelerates alarmingly through the bottom half. The only point a typical gauge is actually calibrated at is empty. Which leads to the two oldest rules in fuel practice: dip the tanks before flight, because the dipstick is the one instrument nobody argues with — and treat the gauge's opinion of the top half as entertainment.
The tank, translated into time.
The reserve is drawn as a wall, not a region — every plan that "just dips into it" was a shorter aircraft range wearing a longer number. The dashed wedge is the disagreement between a ±15% gauge and a ±2–3% flow meter about when the tank is actually dry: the wedge is why the wall exists.
Flow meters count, gauges guess.
A fuel-flow totalizer counts every litre through the line to within a percent or two — but it only knows what you told it at fill-up. Mistype the uplift and it will confidently mis-count all day. The gauge is crude but incorruptible: it measures the tank itself. The professional habit is the hourly cross-check: totalizer says, gauge agrees or objects. When they diverge beyond the usual slop, that disagreement is itself information — a leak, a misfuel, a typo — and it is the cheapest warning you will ever get.
Reserves are for the day you meet them.
Regulatory minima differ by country — commonly around 30 minutes for day VFR — but the physical definition serves better: enough fuel to fly to a realistic alternate, join, go around once, and land, with the engine never knowing anything was unusual. Size your reserve to that, not to the regulation, and place the diversion decision at half-tank rather than at reserve: decisions made with fuel in hand are calm ones. The reserve you plan never to use is doing its job precisely by being boring.
In your cockpit.
With flow, totalizer and GPS on one bus, the instrument keeps this whole ledger live: endurance to reserve, range rings that shrink with a headwind, landing fuel at the destination. Let it do the arithmetic — and keep the two habits it cannot do for you: the dipstick before engine start, and the honesty about which number you will believe when they disagree.
The leaning discipline the flow number comes from is the engine monitoring article.