Determining the per cent difference between two numbers gives you an idea of improvement or a decrease in activity over time, especially if comparing grades or business revenues. Keeping track of which number came after the other helps you to determine if the percentage difference indicated an increase or a decrease. Once you know how to calculate a percentage difference between two numbers, apply it to evaluating personal finance, business revenue or economic activity.

Subtract the last recorded number from the first number. For example, if you need to find the per cent difference between the £6 you made yesterday and the £9 you made today, construct the equation 15 -- 10 = £3.

Determine if the difference indicated an increase or decrease in activity. Identify a positive number for the difference as an increase and a negative number as a decrease. For instance, if the difference between the last and the first numbers equalled -5 (10 - 15), the resulting percentage would show a decrease in difference.

Divide the first number by the difference to determine the ratio. For the example: 5/10 = 0.5. Ignore any negative signs during this step, but keep in mind whether the per cent will show a positive or negative trend.

Multiply the ratio by 100 to find the percentage difference. For instance: 0.5 x 100 = 50 per cent, meaning there was a 50 per cent increase in money earned today over yesterday.