How I picked these: how they work, specs and buyer reviews. I haven't owned every one of them.
What to expect
A slide rule and an abacus do different jobs. Know which one you need before you buy.
- A slide rule multiplies and divides. It also does squares, roots and more on rules with extra scales. It can't add or subtract.
- It gives about three figures. On a normal rule you read an answer like 3.14 or 391, not 391.27. That's plenty for most real jobs, but not for money.
- You place the decimal point yourself. The rule gives the digits. You decide whether it's 6, 60 or 0.6 by a quick rough estimate.
- An abacus adds and subtracts, exactly. A soroban (Japanese abacus) is fast once learned. It takes practice: weeks of a few minutes a day, not an afternoon.
Why these
Circular slide rules are still made in Japan by Concise, so you can buy one new. The scales run round in a circle, so the answer never runs off the end, which is the most annoying thing about a straight rule.
The famous straight rules (Faber-Castell, Aristo, Pickett, and British makers like Thornton and Unique) stopped being made when pocket calculators arrived in the 1970s. Millions were made, so used ones on eBay are common.
The slide rule's weak spot is adding up. A soroban fills that gap, and with practice it's as quick as a calculator for long columns of figures.
Some rules are built for one job. The nautical rule below does speed, time and distance for boats. It's the easiest way to see why sailors and pilots kept using rules long after calculators.
Nothing to charge, nothing to update. A rule from the 1950s works exactly as it did then, and it will still work in a drawer in fifty years.
Because you place the decimal point yourself, you always make a rough estimate first. That habit catches mistakes a calculator would let through.
How it works
The trick is logarithms. You don't need to know the maths, only one fact: on a log scale, the distance from 1 to any number is set so that adding two distances multiplies the numbers. The distance to 2 plus the distance to 3 equals the distance to 6.
A slide rule has two identical log scales side by side. The D scale is on the fixed body. The C scale is on the slide in the middle. Slide C along D and you add one length to another. That's a multiplication.
That's also why the marks bunch up towards the right: the gap from 1 to 2 is much bigger than the gap from 9 to 10.
Multiplying
To work out 2 × 3: slide the C scale so its 1 (the left end, called the index) sits over 2 on D. Move the cursor to 3 on C. Read the answer under it on D: 6.
Dividing
Division is the same thing backwards. To work out 6 ÷ 3: put the cursor on 6 on D, slide C until its 3 sits under the cursor, then read D under C's 1. That gives 2. Look at Fig 1 again: it's the very same setting.
The decimal point
The rule doesn't know if you meant 2 × 3, 20 × 3 or 0.2 × 300. All three give the same setting and the same digits, 6. You round the numbers in your head (20 × 3 is about 60) and put the point in the right place. After a week this becomes automatic.
The abacus
A soroban has rods, one per digit. Each rod has one bead above the bar worth 5 and four beads below worth 1 each. A bead counts when it's pushed against the bar. Any digit from 0 to 9 is one push of the fingers, so adding is quick.
What to buy
These are on Amazon UK, checked 7 November 2026. Listings change, so check each one before you order.
These are affiliate links. If you buy through them I make a small commission, at no extra cost to you. It helps cover the cost of running this site. This is a hobby, not a shop.
Slide rules, new
Abacus
Not on Amazon
Learn it
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Learn to read the D scale
Before any sums, just read numbers. Between 1 and 2 the marks usually go up in tenths. Further right they go up in bigger steps because there's less room. Find 1.5, 2.5, 3.14 and 7 on D. Then find them on C.
Why: almost every mistake on a slide rule is a misread mark, not a wrong method. Get fluent at reading first.
-
Multiply 2 × 3
Put C's 1 over D's 2. Cursor to C's 3. Read 6 on D. Then try 2 × 4 without moving the slide: cursor to C's 4, read 8.
Why: once the slide is set to "times 2", every number on C has its double sitting under it on D. That's how people did whole tables of sums with one setting.
-
Divide 6 ÷ 3
Cursor on 6 on D. Slide C's 3 under the cursor. Read D under C's 1: 2.
Why: dividing subtracts one length from the other, so it's the multiply setting read the other way round.
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Estimate, then place the point
Before every sum, round the numbers and do it roughly in your head. For 25 × 3.2, think "25 × 3 is 75". The rule gives the digits 8 and 0. So the answer is 80.
Why: the rule can't tell 8 from 80 or 800. Your estimate can.
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When the answer runs off the end
On a straight rule, try 4 × 5. With C's 1 over D's 4, C's 5 is past the end of D. Instead put C's 10 (the right index) over D's 4, then read D under C's 5: 2, which your estimate tells you is 20.
Why: using the other end shifts the answer by a factor of ten, which you already handle with the decimal point. A circular rule has no ends, so this step disappears.
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Start the soroban
Hold it flat, beads in their resting place: upper beads up, lower beads down. Thumb pushes lower beads up, index finger moves everything else. Set 1 to 9 on one rod, clear it, repeat. Then add single digits on one rod, then carry to the next rod.
Why: the speed comes from your fingers knowing the moves, not from thinking. Short daily practice builds that far faster than a long session once a week.
Practice sums
Do these on the rule and check the last column after. Being within the last digit is a good result.
| Sum | Rough estimate | Exact answer |
|---|---|---|
| 1.5 × 4 | ≈ 6 | 6 |
| 25 × 3.2 | ≈ 75 | 80 |
| 3.14 × 7 | ≈ 21 | 21.98 |
| 17 × 23 | ≈ 400 | 391 |
| 45 × 0.6 | ≈ 30 | 27 |
| 8 ÷ 2.5 | ≈ 3 | 3.2 |
| 90 ÷ 12 | ≈ 8 | 7.5 |
Using it
- Cooking and DIY. Scaling a recipe from 4 to 6 people is "times 1.5": set it once and read every ingredient off. Same for converting units, like inches to centimetres (times 2.54).
- Walking and sailing. Speed, time and distance is one setting on the nautical rule. Distance on the map, your walking speed, read the time.
- Keep it clean. Wipe with a barely damp cloth and dry it. Don't use solvents or cleaning sprays: many rules are plastic or celluloid and the printing comes off.
- Store it flat, out of the sun. Heat and sunlight can warp a straight rule and yellow the scales. Keep it in its case.
- Soroban: little and often. Ten minutes a day of simple adding beats an hour at the weekend.
Common problems
| What you see | Likely cause | Fix |
|---|---|---|
| Answer is right digits, wrong size | Decimal point in the wrong place | Estimate first, every time. Round to one figure and multiply in your head. |
| Answer is off past the end of D | Used the left index when the right one was needed | Put C's 10 over the first number instead of C's 1. |
| Answers are slightly out, always the same way | Cursor line or scales out of line | Line up C1 with D1 and check the cursor crosses both cleanly. Many cursors have small screws to adjust them. |
| Slide is stiff or jerky | Dirt in the grooves, or a warped vintage rule | Clean the grooves with a dry cloth. A warped rule can't really be fixed; ask the seller about this before buying. |
| Can't read between marks | Normal at the right-hand end | Judge by eye between marks. A longer rule, or a circular one, gives more space. |
| Soroban beads slide on their own | Held at an angle, or very loose beads | Work on a flat table. Clear by running a finger along the bar before each sum. |
Notes
The slide rule came from John Napier's logarithms in the early 1600s, and William Oughtred is usually credited with putting two log scales side by side to slide against each other. The method above is the standard one from any slide rule manual. Old manuals from Faber-Castell, Pickett and others are often scanned and shared by collector groups, and they're the best way to learn the extra scales.
The "about three figures" accuracy is for a normal 25 cm rule. Longer and circular rules give a little more.
Related guides: needle meters, compasses and sundials, mechanical watches, Morse code without batteries.