How is prorate calculated?

When you upgrade a plan at any given time, the system will calculate the prorated price to make sure you're not overpaying. 

Per-day amount calculation:

First, the system will calculate the per-day amount owed by the customer for the lower-tier (current) plan. This per-day amount is then multiplied by the number of days remaining in that billing cycle, and we get the amount that will be refunded to your account in the form of a credit note. 

For example: 

Let's say you are on the Basic plan ($10.00 / month). You started the subscription on September 1st but decided to upgrade to Pro plan ($99.00 / month) on September 15th, so the numbers will look like this:

Total days in the billing cycle: 30 days

Basic plan: $10.00 / 30 = $0.33 / day

Days remaining on the billing cycle: 30 - 15 = 15 days

Refund (for Basic plan): $0.33 * 15 = $4.95

Upon upgrade calculation:

Next, the system will calculate the per day charge of the higher-tier (new) plan and multiply by the number of days remaining in that billing cycle. This is the additional amount that you will be paying for the new plan.

Using the same example above:

Pro plan: $99.00 / 30 = $3.30 / day

Upgrade charge (for Pro plan): $3.30 * 15 = $49.50

Given this example, you will get a $4.95 refund in the form of a credit note, and you'll be charged $49.5 upon upgrade.