Prorated Invoice: How to Calculate Pro Rata Charges
How to make a prorated invoice: pro rata calculation methods for rent, subscriptions and retainers, daily rate formulas, worked examples and common mistakes.
Quick answer: A prorated invoice charges only for the part of a billing period actually used. To calculate it, find the daily rate (the period price divided by the days in that period) and multiply it by the days used. For example, $1,500 monthly rent for 13 days of a 30-day month is $1,500 ÷ 30 × 13 = $650.
Proration comes up whenever a recurring charge does not line up neatly with your billing cycle: a tenant moving in mid-month, a client upgrading a subscription, a retainer starting on the 15th or a contract ending early. The maths is simple, but the method you choose changes the answer, so it is worth agreeing it up front. Once you have the figure, our free invoice generator lets you show the full price, the period covered and the prorated amount on one line.
What does prorated mean?
Prorated comes from the Latin pro rata, "in proportion". A prorated charge is the normal price scaled down in proportion to the time used. You will see it written as prorated, pro-rated, pro rata or pro-rata; they all mean the same thing. Proration can also run in reverse, as a prorated credit when a customer leaves part-way through a period they have paid for.
When to use a prorated invoice
- Rent: a tenancy that starts or ends mid-month. See our rent invoice template guide.
- Subscriptions and SaaS: a customer joins mid-cycle, upgrades, downgrades or adds seats.
- Retainers: a monthly retainer that starts or ends part-way through the month. See retainer invoices.
- Memberships and services: gyms, cleaning contracts, managed IT or equipment rental.
- Aligning billing dates: moving a client from billing on the 18th to billing on the 1st.
Pro rata calculation: the basic formula
Prorated amount = (period price ÷ units in the period) × units used
The "units" are usually days, but can be working days, hours or months. The choice of how many days are "in the period" is where methods differ.
Three ways to calculate a daily rate
| Method | Daily rate formula | Pros | Cons |
|---|---|---|---|
| Actual days in the month | Monthly price ÷ 28, 29, 30 or 31 | Exact for that month; most common | Daily rate changes month to month |
| Annual (365-day) method | Monthly price × 12 ÷ 365 | Same daily rate all year | Slightly different from actual-days total |
| Banker's month (30 days) | Monthly price ÷ 30 | Simple, predictable | Over- or under-charges in 31-day months and February |
| Working days | Monthly price ÷ working days in month | Fair for weekday-only services and retainers | Needs a holiday calendar |
None of these is "right" in general. Pick one, write it into the contract or lease, and use it every time.
Daily rate by month length: $1,000 monthly price
| Month length | Actual-days daily rate | Annual-method daily rate |
|---|---|---|
| 28 days (February) | $1,000 ÷ 28 = $35.71 | $1,000 × 12 ÷ 365 = $32.88 |
| 29 days (February, leap year) | $1,000 ÷ 29 = $34.48 | $32.88 |
| 30 days | $1,000 ÷ 30 = $33.33 | $32.88 |
| 31 days | $1,000 ÷ 31 = $32.26 | $32.88 |
The table shows why a customer who starts in February pays noticeably more per day under the actual-days method than one who starts in March. Neither is wrong; it is a consequence of months having different lengths.
Prorated rent example
A tenant moves in on 18 June. Rent is $1,500 per month. June has 30 days, and counting 18 June to 30 June inclusive gives 13 days.
- Actual days: $1,500 ÷ 30 = $50.00 per day. $50.00 × 13 = $650.00.
- Annual method: $1,500 × 12 = $18,000 per year; $18,000 ÷ 365 = $49.315 per day; $49.315 × 13 = $641.10.
The $8.90 difference is why the method should be in the lease. The invoice line might read: "Rent, 18 June to 30 June 2026 (13 of 30 days at $1,500/month), $650.00", followed by a second line for July's full rent if you bill them together.
Prorated subscription example: mid-cycle upgrade
A customer on a $30 per month plan upgrades to a $60 plan on day 11 of a 30-day billing cycle, leaving 20 days.
- Credit for unused time on the old plan: $30 ÷ 30 × 20 = $20.00.
- Charge for remaining time on the new plan: $60 ÷ 30 × 20 = $40.00.
- Net prorated charge: $40.00 − $20.00 = $20.00.
Show both lines on the invoice rather than just the net figure, so the customer can see the credit and the new charge. From the next cycle they pay the full $60. For a downgrade, the same calculation produces a net credit, which you would carry forward or document with a credit note.
Prorated retainer example using working days
A consultant's retainer is $4,000 per month for weekday availability. The client signs part-way through a month with 22 working days, and 12 working days remain.
$4,000 ÷ 22 = $181.818 per working day; $181.818 × 12 = $2,181.82 after rounding to the cent. Keep full precision until the final step: rounding the daily rate to $181.82 first would give $2,181.84.
Prorating annual contracts and cancellations
Proration works the same way for longer periods. Suppose a client pays $2,400 up front for a 12-month maintenance plan and cancels after 4 full months, and your terms allow a refund for unused whole months.
- Monthly value: $2,400 ÷ 12 = $200.
- Unused months: 12 − 4 = 8.
- Prorated refund: $200 × 8 = $1,600.
Document the refund with a credit note that quotes the original invoice number, rather than editing the original invoice. If your terms include a cancellation fee, show it as a separate line so the client sees the full calculation. If you do not refund partial periods, say so clearly in the contract and on the original invoice.
Proration also helps when you move a client to a new billing date. If a client signs on the 18th but you bill everyone on the 1st, issue one prorated invoice for the 18th to the end of the month, then full invoices from the 1st. Keeping every client on the same cycle makes recurring invoices and cash flow planning simpler.
How to show a prorated charge on an invoice
- Period covered, with start and end dates.
- Full period price for reference.
- Calculation basis, such as "13 of 30 days" or "12 of 22 working days".
- Prorated amount as the line total.
- Tax on the prorated amount, not the full price. The sales tax calculator works on any amount.
A clear line removes the most common question customers ask about a prorated invoice: "why is this not the usual amount?"
Prorating hourly or daily work
If you already bill by the hour or day, you rarely need proration: you simply bill the hours worked. Proration is for fixed-price recurring charges. When a client asks to convert a fixed fee into a partial month, it can be cleaner to bill actual time instead; see how to invoice for hourly work.
Prorated vs pro rata: other places you will see the terms
The same arithmetic appears well beyond invoices, which is useful when a client asks you to explain your figure:
- Prorated salary: employers scale an annual salary to a part-time schedule or a partial first month. The logic is identical to a prorated retainer.
- Prorated rent deposits and service charges: landlords and property managers apportion charges between outgoing and incoming tenants.
- Insurance premiums: a policy cancelled mid-term may be refunded pro rata, sometimes less a fee.
- Shared costs: a utility bill split between two periods, or between two clients who shared a resource, is apportioned pro rata by days or usage.
Whatever the context, the questions are the same: what is the full-period price, how is the period measured, and how many units were actually used? Write those three answers on the invoice and most queries disappear before they are asked.
Common proration mistakes
- Not agreeing the method, then arguing over a few dollars.
- Forgetting to count the start or end day.
- Using 30 days for every month without telling the customer.
- Rounding the daily rate before multiplying.
- Applying tax to the full monthly price rather than the prorated amount.
- Showing only a net figure for upgrades, which hides the credit.
Prorated invoice checklist
- Proration method written into the contract or lease.
- Days or working days counted inclusively and checked against a calendar.
- Daily rate kept at full precision; final amount rounded once.
- Period, basis and full price shown on the invoice line.
- Tax calculated on the prorated amount.
- Next full-period invoice scheduled on the new cycle date.
Next step: create your prorated invoice
Work out the amount with the formula above, then open the invoice generator, add a line describing the period and calculation basis, and enter the prorated total as the rate with a quantity of 1. Download the PDF and set a reminder for the first full-period invoice; a monthly reminder or recurring workflow keeps that schedule on track.
Frequently asked questions
What does prorated mean on an invoice?
Prorated means the charge has been scaled to cover only part of a normal billing period. If a monthly service starts halfway through the month, a prorated invoice bills roughly half the monthly price. The invoice should show the period covered and how the amount was calculated.
Do I count the start day and the end day when prorating?
Usually you count both, because the customer has the service on the day it starts and on the last day of the period. For a period from 18 June to 30 June that gives 13 days. State in your contract whether days are counted inclusively so customers can check your figure.
Should I round prorated amounts up or down?
Calculate with full precision and round only the final amount to the nearest cent or penny. Rounding the daily rate first can shift the total by several units on longer periods. Whatever you choose, apply it consistently and follow any rounding rule in your contract.
How do I prorate a refund when a customer cancels?
Work out the unused days in the period they already paid for, multiply by the daily rate and issue a credit note or refund for that amount, if your terms allow refunds. Some businesses do not refund partial periods at all, so state your cancellation policy clearly in the contract.
Is prorated rent required by law?
It depends on the jurisdiction and the lease. Many leases set out how partial months are charged, and some local rules affect how rent is calculated when a tenancy starts or ends mid-month. Check the lease and local landlord-tenant rules before deciding which method to use.
Should I bill a prorated amount in advance or in arrears?
Follow the same timing as the normal charge. If you bill monthly in advance, invoice the prorated part period when the service starts, often together with the next full month. If you bill in arrears, include the partial period on the first regular invoice. Showing both lines on one invoice reduces admin and makes the calculation easy for the customer to check.
How do I prorate with the annual method in a leap year?
Decide whether your contract uses 365 days every year or the actual days in the year. In a leap year the actual-days version divides by 366, which gives a slightly lower daily rate. Either approach is fine if it is written down and applied consistently. Mixing them between customers or years is what causes disputes over small differences.
Can I charge a full month instead of prorating?
Yes, if your contract or terms say so. Many subscriptions and memberships charge the full period regardless of start date, or start the first billing period on the sign-up day so no proration is needed. What you cannot do is charge a full month when the agreement promised proration. Consumer protection rules may also apply, so state your policy clearly before the customer signs.
How do I prorate a price increase that starts mid-month?
Split the month at the effective date. Bill the days before the change at the old daily rate and the days from the change at the new daily rate, then add the two amounts. Show them as two lines with dates, such as 1 to 14 March at the old price and 15 to 31 March at the new price, so the customer can see exactly when the increase applied.
How do I prorate an annual or quarterly fee for a partial period?
Use the same formula with a longer period. Divide the annual fee by 365, or the quarterly fee by the days in that quarter, then multiply by the days used. Some contracts prorate annual fees by whole months instead, dividing by 12. Whichever method you use, state the period, the basis and the full fee on the invoice line.
Invoice Creator Editorial Team
Guides on invoicing, cash flow and small-business finance from Invoice Creator, operated by Next Software Development Company. Content is reviewed for accuracy against the fields our own invoice generator produces.
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