With the 50/30/20 rule, a net income of 1800 a month splits into 900, 540 and 360: one half, three tenths and one fifth of the total. The three shares add up to exactly 1800, with nothing left over and nothing missing. The calculation is a multiplication by fractions, and you check it by adding.
It is a rough guide for dividing a figure, not a rule. If you would rather split by weights (5:3:2) instead of percentages, use the proportional sharing calculator. The amounts on this page have no currency: they work in whichever one you use.
Turn the percentages into fractions
A percentage is a fraction with a denominator of 100: Wikipedia’s Percentage article, for instance, shows 45% as 45/100. To convert, write the percentage over 100 and simplify. To see the reduction step by step, use simplify fractions, and to convert any value, percent to fraction.
| Part | Out of 100 | Simplified | Divide by |
|---|---|---|---|
| Needs | 50/100 | 1/2 | 50 |
| Wants | 30/100 | 3/10 | 10 |
| Savings | 20/100 | 1/5 | 20 |
The names in the first column are the ones most often used for the three groups. As an example, needs usually include rent, groceries and transportation; wants, entertainment, travel and dining out; savings, an emergency fund, money set aside for a trip and contributions to an investment. The arithmetic does not depend on that sorting. What matters is that each row reduces with a different divisor and that the last two cannot be simplified any further.
Worked example: splitting 1800 a month
You earn 1800 net a month and want to split it with the 50/30/20 rule. How much goes to each part?
- Half. 1800 × 1/2 = 900.
- Three tenths. 1800 × 3/10 = 5400/10 = 540.
- One fifth. 1800 × 1/5 = 360.
- Add the three shares. 900 + 540 + 360 = 1800.
Result: 900, 540 and 360.
Check: 30% + 20% is the same as 50%, so 540 + 360 must equal 900. It does.
Each share is figured on the full total. The fraction of a number calculator repeats that multiplication with any amount, and the percentages calculator is useful for checking each share as a percentage.
Check that the three parts add up to the total
A split is valid when its fractions add up to 1. With a common denominator of 10 you get 1/2 + 3/10 + 1/5 = 5/10 + 3/10 + 2/10 = 10/10 = 1. To go from 1/2 to 5/10 and from 1/5 to 2/10, you multiply the numerator and denominator by the same number, so the value does not change.
That sum explains why the shares rebuild the total, whatever the amount: multiplying by each fraction and adding is the same as multiplying by 1. So the final check in the example, 900 + 540 + 360, never fails if the multiplications are right. If it fails, one of the three has an error, and it is not fixed by adjusting a share by eye.
Apply the same fractions to another total
If the income changes from one month to the next, the fractions stay the same and only the number you multiply changes. The table shows three sample totals.
| Total | Half | Three tenths | One fifth |
|---|---|---|---|
| 1450 | 725 | 435 | 290 |
| 2000 | 1000 | 600 | 400 |
| 1005 | 502.5 | 301.5 | 201 |
The first two rows give whole numbers because 1450 and 2000 are multiples of 10. The third is not, which is why decimals ending in .5 appear. When the total does not divide exactly, the share stays as a fraction or decimal and is rounded at the end, if needed. Adding two months before splitting, or splitting each month and adding the shares, gives the same result, thanks to the distributive property of multiplication.
Swap 50/30/20 for another split
Any split works if its fractions add up to 1. With 60/25/15 the simplified fractions are 3/5, 1/4 and 3/20, and with a common denominator of 20 you get 12/20 + 5/20 + 3/20 = 20/20. On 2400 that gives 1440, 600 and 360, which add up to 2400.
A split like 60/30/20 adds up to 110%, more than the total, and divides nothing. Checking that the percentages add up to 100% is the first test before you calculate.
The 50/30/20 can also be written as the ratio 5:3:2, because 5 + 3 + 2 = 10 and each weight divided by 10 gives the same fraction. With 1450, each unit is worth 145, and the shares are 725, 435 and 290, the same as in the table. The proportional sharing calculator works directly with weights, and the guide to sharing with fractions develops the method. To see why percentage, fraction and ratio describe the same thing, read ratio, proportion and fraction.
Practice
- Split 2500 with the 50/30/20 rule.
- Split 3000 with a 70/20/10 split.
- Split 800 in the ratio 4:3:1.
Show the answers
- 1250, 750 and 500 (they add up to 2500).
- 2100, 600 and 300 (7/10 + 1/5 + 1/10 = 1; they add up to 3000).
- 400, 300 and 100 (4 + 3 + 1 = 8 weights of 100 each).
What the calculation does not settle
The arithmetic of the split is exact, but choosing those three fractions is a convention. The calculation does not say how much it makes sense to save or where to put each doubtful expense, and it is not financial advice. For other uses of proportions, see the fraction applications.
Sources
- Wikipedia, Percentage: a percentage as a number or ratio expressed as a fraction of 100, with 45% equal to the fraction 45/100 (consulted October 17, 2026).