Why Calculate Percentages Mentally?
Even in the age of smartphones and calculators, the ability to perform simple percentage calculations in your head is enormously practical. When shopping, in restaurants, during salary discussions, or when quickly estimating costs -- those who can calculate percentages mentally make better decisions faster.
Trick 1: The 10% Method (Anchor Trick)
Calculate 10% of any number by moving the decimal point one place to the left. 10% of 350 = 35. 10% of 89.99 = 8.999 (about EUR 9).
This trick is the anchor point for almost all other calculations because you can derive from it.
Trick 2: 5% = Half of 10%
Simply halve your 10% result. 5% of 350 = 35 / 2 = 17.50. 5% of 89.99 = approx. EUR 4.50.
Trick 3: 1% = Move Decimal Two Places
1% of 350 = 3.50. 1% of 89.99 = 0.90. With this, you can compose any percentage.
Trick 4: 15% = 10% + 5%
Add the partial results. 15% of 350: 35 + 17.50 = 52.50.
Trick 5: 20% = Double 10%
20% of 350 = 35 x 2 = 70. Alternatively: divide by 5 (350 / 5 = 70).
Trick 6: 25% = Divide by 4
25% of 350 = 350 / 4 = 87.50. That's one quarter of the value.
Trick 7: 19% VAT = 20% Minus 1%
The most important trick for German everyday life: 19% of 350 = 70 (20%) minus 3.50 (1%) = 66.50.
Or in reverse for net-to-gross: Gross is approximately 'net plus one fifth'. From 100 net: 100 + 20 = 120 EUR (close to the exact value of 119 EUR).
Bonus: The Swap Trick
8% of 25 = 25% of 8 = 2. Because multiplication is commutative, you can swap the numbers when one calculation is easier. 4% of 50 = 50% of 4 = 2. This trick always works and can drastically simplify calculations with awkward numbers.