Double it and add thirty

You are somewhere the numbers are the other way round, and the forecast says a figure that means nothing to you. The real sum is not hard, it is just too many steps to do while somebody is talking to you.

Double the Celsius and add thirty.

That is it. Twenty becomes seventy — the true answer is sixty-eight. Ten becomes fifty, which is exact. Thirty becomes ninety, against a true eighty-six. Across the range of temperatures a day actually is, you are within a few degrees, and a few degrees is not the difference between taking a coat and not.

Going the other way: subtract thirty, then halve. Seventy becomes twenty.

The rule drifts upward as the number climbs, and there is exactly one place that matters. It is above this page, and it is worth reading before you use this on anybody who is ill.

The pairs worth just knowing

Some people skip the arithmetic entirely and memorise a handful of landmarks, and the landmarks turn out to be surprisingly friendly.

Sixteen degrees Celsius is sixty-one Fahrenheit. Twenty-eight is eighty-two. The digits swap round, which is a coincidence and an extremely useful one.

The other approach is to stop converting altogether and learn what the numbers feel like. There is a rhyme people pass around for this — thirty is hot, twenty is nice, ten is cold, zero is ice — and once it is in your head you have stopped needing Fahrenheit at all, which is the actual goal.

Miles and kilometres

Here there is a famous trick, and a quieter argument about whether you should bother with it.

The trick: consecutive numbers in the Fibonacci sequence — 1, 2, 3, 5, 8, 13, 21, 34, 55 — are almost exactly a miles-to-kilometres pair. Five miles is eight kilometres. Eight miles is thirteen. Thirteen is twenty-one. Twenty-one is thirty-four. Checked properly, five miles is 8.05 km and thirteen is 20.92, so it is genuinely accurate rather than merely charming.

The argument against it, from several people at once, is practical. It only lands on the numbers that happen to be in the sequence, it is poor at the small end, and anything else needs you to break the distance into Fibonacci pieces and add them up — by which point you have done more work than the boring method.

The boring method: multiply by 1.6 for miles to kilometres, or by 0.6 going back. If multiplying by 1.6 is awkward, add half and then add a tenth of the original. Ten miles: add five, add one, sixteen kilometres. The true figure is 16.09.

Two smaller ones. For speed limits, halve the kilometres and add the first digit — 50 km/h becomes 25 plus 5, which is 30 mph against a true 31.07. And many cars have both scales printed on the speedometer, one of them small, which is the answer to this that requires no arithmetic at all.

The honest note

Somebody in these conversations pointed out that none of this is mathematics, it is rearranged arithmetic, and they were right. The value is not that it is clever. It is that these versions survive being done in your head, in a car, in a queue, while somebody is asking you a question — which the correct formula does not.

And for anything where four degrees or a tenth of a mile matters, do it properly. This is a set of tools for being roughly right quickly, which is nearly always what is actually wanted, and once in a while is not.