What is the proration formula?

Prorated amount = Monthly fee × Days used ÷ Basis days. Days used counts both the start and end dates inclusively.

Should I use actual calendar days or a fixed 30-day basis?

Use actual calendar days for rent or salaries where monthly variation is acceptable. Use a fixed 30-day basis for SaaS or subscriptions where you want consistent charges regardless of month length. Follow whatever your contract specifies.

How should fractions below ¥1 be handled?

Floor (round down) is most common and favors the customer, but some businesses use ceiling (round up) or standard rounding. If your contract doesn't specify, floor is the safest choice.

How do I prorate a tax-inclusive amount?

First convert to the pre-tax amount by dividing by 1.1 (for 10% tax), then calculate the proration, and finally add the tax back. This tool handles this automatically when you check the tax-inclusive option.

How are cross-month periods calculated?

When the usage period spans multiple months, each month is calculated separately with its own base days, then the results are summed. This tool automatically shows the monthly breakdown.

What is the difference between daily proration and monthly proration?

Daily proration divides by days, suitable for mid-month starts or ends. Monthly proration divides an annual fee by months. Use daily proration for partial-month billing.

How do I calculate a prorated refund for a subscription?

The refund equals the monthly fee minus the prorated usage amount. For example, if a ¥9,800/month service was used for 15 days, the prorated usage is ¥4,900, and the refund is ¥4,900.

Prorate

Proration Calculator

Calculate prorated charges from a monthly fee and usage period. Useful for mid-month subscription changes, rent adjustments, and partial billing.

Try a sample calculation:

Calculate Prorated Amount

Monthly Fee (¥)

Amount includes tax (10%)

Start Date

End Date

Day Count Basis

Actual Days (calendar month) Fixed 30 Days

Rounding

Floor Ceil Round

Days Used - / - days

Daily Rate ¥-

Prorated Total (excl. tax) ¥0

Consumption Tax (10%) ¥0

Total (incl. tax) ¥0

Create an invoice with this prorated amount

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About This Tool

A proration calculator for splitting monthly charges by the number of days used. Enter the monthly fee, start date, and end date to instantly see the prorated amount. Ideal for subscription cancellations, mid-month sign-ups, and partial-month rent calculations.

Formula

Prorated Amount = Monthly Fee × Days Used ÷ Base Days
Days Used: Both the start and end dates are counted (e.g., Apr 1–Apr 10 = 10 days)
Base Days: Either the actual number of days in the month (28–31) or a fixed 30 days
Rounding options: floor (round down), ceil (round up), or standard rounding

Choosing a Day Count Basis

Actual Days: For lease agreements, utility bills, and payroll where fair apportionment based on month length is preferred
Fixed 30 Days: For SaaS subscriptions, telecom charges, etc., where a uniform calculation regardless of month length is standard

Rounding Options

Floor (Round Down): Drops fractions below ¥1. Most common and customer-friendly
Ceil (Round Up): Rounds up fractions below ¥1. Favors the business
Round (Standard): Rounds fractions ≥ 0.5 up, otherwise down
Always follow your contract terms if they specify a rounding method

Cross-month Calculations

When the usage period spans multiple months, each month is calculated separately with its own base days, then the amounts are summed. Under the "Actual Days" basis, the daily rate differs per month. This tool automatically displays the monthly breakdown.

Calculation Examples

Example 1: SaaS Mid-month Cancellation

Cancelling a ¥9,800/month SaaS on April 15 (usage: Apr 1–15, fixed 30 days):

Days used = 15
Prorated amount = 9,800 × 15 ÷ 30 = ¥4,900

Example 2: Mid-month Rent Move-in

Moving into an ¥80,000/month apartment on April 16 (usage: Apr 16–30, actual days):

Days used = 15, Base days = 30 (April)
Prorated amount = 80,000 × 15 ÷ 30 = ¥40,000

Example 3: Payroll for Mid-month Start

Starting a ¥300,000/month job on February 20 (usage: Feb 20–28, actual days):

Days used = 9, Base days = 28 (February)
Prorated amount = 300,000 × 9 ÷ 28 = ¥96,428 (floor)

Note: If the start date is after the end date, the result will be ¥0. Share the URL to let others verify the same calculation.

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