10 Mental Math Tricks Every Student Should Know

Mental math isn’t about showing off—it’s about comfort with numbers. When you can estimate quickly and calculate the basics in your head, homework gets faster, tests feel calmer, and real-life decisions (like discounts or tips) take seconds.

1) Rounding and Compensation

Round one number to an easy value, compute, then compensate. Example: 49 × 6 ≈ (50 × 6) − 6 = 294.

2) Split Multiplication

Break numbers into parts. Example: 23 × 7 = (20 × 7) + (3 × 7) = 140 + 21 = 161.

3) Friendly Pairs for 100

Keep pairs like 25–75 or 40–60 in mind for fast percent and addition work.

4) Doubling and Halving

Useful for products: 25 × 16 = (50 × 8) = 400.

5) 9’s Trick

Multiplying by 9? Think 10× minus the number: 9 × 27 = 270 − 27 = 243.

6) Percent in Your Head

10% is one decimal shift. 20% is double that. 15% is 10% + half of 10%.

7) Cross-Multiplying for Ratios

If a/b = c/d, then ad = bc. Helps compare fractions quickly.

8) Square Near a Base

For numbers near 100: 98² = (100 − 2)² = 10,000 − 400 + 4 = 9,604.

9) Fast Differences

(a − b)² = a² − 2ab + b² gives quick checks and mental estimates.

10) Estimate First, Exact Second

Always predict the ballpark. You’ll spot mistakes and move faster overall.

Try It

Estimate 38% of 250: 10% = 25 → 30% ≈ 75; 8% ≈ 20. Total ≈ 95.

Takeaway: Pick two tricks and use them for a week. Mental math grows from small, consistent practice.