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Pro rata calculator

Avoid mistakes when calculating proportions
Marc Fletcher
Updated 13-08-2022
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Marc Fletcher
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Insert a Full Amount, Full Quantity & Part Quantity in the table.

The correct Part Proportion & Part Amount will be calculated for you
Full Amount
Full Quantity
Part Quantity
Part Proportion
Part Amount
1
25
30.00
10.00
33.33%
8.33
There are no rows in this table
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Parts explained as examples:

Full Amount. Say the cost of a Netflix subscription for 1 month. E.g. $14.99
Full Quantity. It’s reasonable to divide a month into days. E.g. November is 30 days
Part Quantity. A number representing part of the Full Quantity. E.g. 18 days of November
Part Proportion. The percentage which Part Quantity is of Full Quantity. E.g. 18 days / 30 days = 60%
Part Amount. The Part Proportion percentage applied to the Full Amount: E.g. 60% of $14.99 =$8.99

What’s the point of this?

Let’s learn about fair proportions when we need to calculate, say, how much of something has been used
Imagine you subscribe to Netflix for 1 month, but cancelled part-way through the month. You paid for 1 month but only used the service for part of the month. How much should you be refunded?
Let’s say the subscription cost is $14.99 per month and is paid upfront. Full Amount: $14.99.
We’re calculating a proportion of something. In this case, the refund is a proportion of the Full Amount after cancellation. You should pay for the proportion of the month before cancellation and be refunded for a proportion of the month after cancellation. It’s convenient to use days as equivalent parts of a month. For our scenario we’ll use the month of November; it has 30 days. Full Quantity: 30.
Say after 18 days, you cancel the subscription. Part Quantity: 18.
To calculate the proportion of days before cancellation, divide the Part Quantity (which is 18 days) by the Full Quantity (which is 30 days).
Part Proportion = Part Quantity / Full Quantity Part Proportion = (18 / 30) Part Proportion = 0.6 or 60%
You used Netflix for 60% of the time you paid for. In other words, the Part Quantity is 60% of the Full Quantity. Therefore, we can also legitimately conclude the Part Amount is 60% of the Full Amount.
Let’s use the Part Proportion to calculate how much of the Full Amount we should fairly pay.
Part Amount = Full Amount x Part Proportion Part Amount = $14.99 x 0.6 Part Amount = £8.99
You should pay $8.99 for 18 days use of Netflix.
When you signed up, you paid $14.99 which means you’ve overpaid. We can calculate the refund amount by deducting the amount you should pay from the amount you have already paid
Refund = Full Amount - Part Amount Refund = $14.99 - $8.99 Refund =$6.00

Summary

A less verbose way of performing a pro rata calculation:
Part Amount = (Full Amount / Full Quantity) x Part Quantity Part Amount = (14.99 / 30) x 18 Part Amount = 8.99

Notes.
- To include a percentage in a calculation we convert it to a decimal by dividing the percentage number by 100. Therefore 60% converted to a decimal: 60/100 = 0.6.
- The example above uses November 1st as the start date for the subscription.

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