By Contributor (Updated Aug 30, 2022)

Mathematics is woven into everyday life—from tuning instruments to drafting blueprints. Mastering simple number properties, such as divisibility by 9, can save time and sharpen analytical skills. Below is a clear, step‑by‑step method that relies only on basic addition.

Step 1 – Add the Digits

Start by summing every digit in the number. For example, 189 becomes 1 + 8 + 9 = 18.

Step 2 – Reduce the Sum

If the result is greater than 10, add its digits together again: 18 → 1 + 8 = 9.

Step 3 – Repeat Until a Single Digit

Continue this process until only one digit remains. In the example, the final digit is 9.

Step 4 – Check the Result

The original number is divisible by 9 only when the single-digit sum equals 9. Thus, 189 is indeed divisible by 9.

TL;DR

Use this quick digit‑sum trick with any integer to instantly verify divisibility by 9.