Circular Slide Rule for the Apple Watch.
The aviator's bezel slide rule, back on your wrist. Also on iPhone and iPad.
No more punching calculator buttons on your watch.
Two concentric scales, one ratio. Everything else follows from that.
Both rings are logarithmic, and both cover one decade — 10 round to 100 in a single turn. Line up any number on the outer ring with any number on the inner ring and you have fixed a ratio. Every other pair of numbers round the face now holds that same ratio, in both directions, all at once.
That is the whole mechanism. A speed problem, a fuel problem and a multiplication are the same act performed on different labels.
- Outer ringturns
- Rate, distance, fuel, quantity — whatever the answer is measured in. This is the ring you move, with a finger or the Digital Crown.
- Minutes ringfixed
- Time in minutes, and the ring that carries all three indices. It never moves, so the mark you read against is always in the same place.
- Hours ringfixed
- 1:00 to 10:00, beneath the minutes, so a long leg reads as 2:30 rather than 150 minutes.
Speed, time and distance.
Turn the outer ring until your groundspeed sits on the amber rate index — the 60 mark at twelve o'clock. That is the entire setup, and you do it once.
The dial is now a table of that speed. Any time on the minutes ring reads its distance on the outer ring, and any distance reads its time. You are not stepping through a calculation; you are looking one up.
- Set 120 (knots) on the amber 60 index.
- Read any time against any distance, in either direction.
| Minutes ring | Outer ring |
|---|---|
| 30 | 60 nm |
| 45 | 90 nm |
| 80 (1:20 on the hours ring) | 160 nm |
| 50 | 100 nm |
The last row is the same setting read backwards: you wanted to know how long 100 nm takes, and it was already on the face.
Fuel is the same setting.
Nothing about the method changes when the units do. Put your burn rate on the rate index instead of your speed, and the outer ring reads gallons where it read miles.
- Set 9.5 (gallons per hour) on the amber 60 index.
- 40 minutes reads 6.3 gallons.
- Going the other way, 38 gallons reads 4:00 on the hours ring — your endurance at that burn.
Any per-hour rate works the same way: litres, pounds, miles, anything. The dial does not know or care which.
Multiplying.
Here the rings stop being speed and time and go back to being plain numbers. The mark you work against is the red 10 — the unit index — because setting a number over 10 fixes the ratio at that number divided by ten.
- Turn the outer ring until 12 sits over the red 10 index.
- Find the multiplier, 8, on the inner ring — that is the graduation marked 80, since the ring is labelled across one decade and a mark stands for its digits at any size.
- The answer is opposite it on the outer ring: 96.
The setting holds for every multiplier at once, so with 12 on the index the face has already worked out 12 × 3, 12 × 7 and 12 × 8. Slide along the inner ring and read them off.
Dividing.
Division is the same move in reverse. Instead of setting the number against the index, you set it against its divisor — and then the index is where the answer appears.
- Turn the outer ring until 96 sits over 8 on the inner ring.
- Read the answer on the outer ring, over the red 10 index: 12.
Another: put 450 over 18 and the 10 index reads 25. This is also how you recover a rate you did not start with — 450 miles over 18 gallons is 25 miles per gallon, and the same setting will now answer every other fuel question at that figure.
Minutes into seconds.
The third mark, the teal 36, is there because an hour holds 3,600 seconds. It converts between minutes and seconds without disturbing anything else about how the dial is read.
- Put the figure in minutes — say 2.5 — on the amber 60 index.
- Read the outer ring against the teal 36 index: 15, which is 150 seconds.
That shift from 15 to 150 is not a quirk of this example. It is the one thing a slide rule will never do for you, and it has its own section below.
The decimal point is yours.
One turn of the ring covers one decade, so a single graduation stands for its digits at any magnitude. The mark reading 12 is 1.2, 12, 120 and 1,200 — the dial has no way to tell them apart and does not try. It is also why the inner ring, labelled from 10 round to 90, is where you find 8 and 0.8 as well as 80.
In practice this costs nothing, because you already know roughly what the answer should be. Twelve gallons an hour for forty minutes is somewhere near eight gallons, so when the face reads 80 you know to call it 8.0 and not 80 or 0.8. Estimate first, read the digits second.
The habit worth keeping: work out the order of magnitude in your head before you look at the dial. If the digits you read do not fit the estimate, you have set the wrong ring, not found a surprising answer.
In the hand.
Turn the ring with a finger, or with the Digital Crown on Apple Watch. Let go mid-turn and it carries on and coasts to a stop. Turn slowly and the crown gets finer under your finger, so you can settle on the graduation you meant.
A click marks each numbered graduation passing the index — feedback, not a detent, so the dial never pulls you onto a mark you did not choose. A complication puts the face one tap from the watch face, and it opens where you left it.
On iPhone and iPad it is the same instrument at a size that has room for the numerals the watch has to leave off: same graduations, same geometry, more type.
What it does not do.
There is no equals key, and that is the point rather than an omission. A slide rule shows you the whole family of answers at once and lets you choose; a calculator shows you one number and forgets it.
- No wind side. This is the calculator face only. Headings and wind correction are not part of it.
- No typed entry, no computed readout. You set the ring and you read the scales.
- No account, no analytics, no advertising. Nothing is collected and nothing leaves your device.
Two things are remembered on the device and nowhere else: the angle you last left the dial at, and when. Both are discarded after a day. See the privacy policy.
About the E6B.
The E6B is an analogue flight computer worked out in the 1930s by Philip Dalton, a lieutenant in the U.S. Naval Reserve, for training and for figuring problems in the air. Generations of pilots learned on one, and the circular slide rule on its front has barely changed since: multiplication and division, time, speed and distance, and fuel burn, all on one face.
This app is that face and nothing else. If you wore an aviator watch and miss reaching for its bezel, the bezel is what this is.
Getting in touch.
Bugs, questions and suggestions all go to one place: [email protected]. Tell me which device you were on and what the dial was showing, and I will get back to you.
Why doesn't the clock go away on my watch? watchOS will not let an app hide it. The face is laid out to sit clear of it.
Not a certified flight computer. Aviator Watch E6B Slide Rule is a reference and training tool. Verify critical calculations independently.