What is a complement on the abacus?

When a rod doesn’t have enough lower beads to add a number directly, you use its complement. To add 3 to a rod showing 2, bring down the five-bead (+5) and remove 2 lower beads (−2): 2 is 3’s complement of 5. When the total passes 9 you use the complement of 10: to add 9, take away 1 here and carry 1 to the next rod on the left.

What’s the difference between a soroban and a suanpan?

Each rod of a soroban has one upper bead worth 5 and four lower beads worth 1, which is exactly enough for 0–9. The Chinese suanpan has two upper and five lower beads, so one rod can temporarily hold up to 15 — handy in traditional division methods and old weight units based on 16. The modern soroban is simpler and is what most mental-arithmetic courses use.

Why does the demo start from the left?

Abacus users work from the highest place to the lowest, the opposite of written column addition. Reading and moving beads in the same direction is faster, and a carry just adds 1 to a rod you’ve already set. The answer is the same as on paper; carry and borrow moves are listed as indented extra steps.

How should my child practise flash anzan?

Start with single-digit numbers, 3 to 5 at a time, about 1.5–2 seconds each. As it gets easy, speed up, add more numbers and then more digits. Beginners can move the beads on the abacus below as each number appears; later, the goal is to picture the beads in your head — that’s mental abacus (anzan).

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📿Online Abacus - Soroban & Suanpan with Step-by-Step Rules

For children learning the abacus or mental arithmetic, and for anyone who wants to see exactly how the beads move. Choose a Japanese soroban (one upper bead, four lower) or a Chinese suanpan (two upper, five lower, so a rod can hold up to 15 for a while), from 3 to 17 rods, with an optional number of decimal places — the ones rod is marked with a red dot, there’s a unit dot every three rods, and place names are written underneath. Tap a bead to move it to the beam or away from it, or drag it up and down, and the value updates live; type a number to set it in one go. The step-by-step demo adds or subtracts a second number from left to right the traditional way, and every move is labelled with its method — direct, complement of 5, complement of 10, or both — including the extra moves caused by carrying and borrowing, with the traditional Chinese rhyme for each (all 52 rhymes are covered). “Set the number” shows a number for your child to put on the abacus and scores it automatically, and flash anzan flashes numbers one after another at the speed you choose, then asks for the total — use the abacus underneath while you’re learning.

Free · No sign-up · Runs in your browserLast updated: 2026-10-26

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How to use Online Abacus

1Choose a soroban or suanpan, the number of rods and decimal places
2Tap or drag beads, or type a number to set it
3Enter two numbers in the demo to step through each move, then practise with the drills

FAQ

What is a complement on the abacus?: When a rod doesn’t have enough lower beads to add a number directly, you use its complement. To add 3 to a rod showing 2, bring down the five-bead (+5) and remove 2 lower beads (−2): 2 is 3’s complement of 5. When the total passes 9 you use the complement of 10: to add 9, take away 1 here and carry 1 to the next rod on the left.
What’s the difference between a soroban and a suanpan?: Each rod of a soroban has one upper bead worth 5 and four lower beads worth 1, which is exactly enough for 0–9. The Chinese suanpan has two upper and five lower beads, so one rod can temporarily hold up to 15 — handy in traditional division methods and old weight units based on 16. The modern soroban is simpler and is what most mental-arithmetic courses use.
Why does the demo start from the left?: Abacus users work from the highest place to the lowest, the opposite of written column addition. Reading and moving beads in the same direction is faster, and a carry just adds 1 to a rod you’ve already set. The answer is the same as on paper; carry and borrow moves are listed as indented extra steps.
How should my child practise flash anzan?: Start with single-digit numbers, 3 to 5 at a time, about 1.5–2 seconds each. As it gets easy, speed up, add more numbers and then more digits. Beginners can move the beads on the abacus below as each number appears; later, the goal is to picture the beads in your head — that’s mental abacus (anzan).