Accountants Invoice Total (Monthly) Formula
Learn how a monthly accounting invoice total is calculated from a retainer, extra work, expenses, discount and VAT.
This calculation estimates the total amount to invoice for a month. It builds a subtotal from the agreed retainer, extra billable work and rechargeable expenses, applies any discount, then adds VAT to the discounted net amount.
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Total invoice amount
Where:
Add the retainer, additional work and expenses. Reduce that subtotal by the discount, then add VAT to the discounted amount.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| monthlyRetainer - Monthly retainer fee | The agreed fixed monthly fee before VAT. | currency |
| additionalHours - Additional billable hours | Hours worked outside the monthly retainer. | hours |
| hourlyRate - Additional work hourly rate | The fee charged for each additional hour. | currency |
| rechargeableExpenses - Rechargeable expenses | Approved expenses being passed on to the client. | currency |
| discountRate - Discount rate | The percentage discount applied to the subtotal before VAT. | percent |
| vatRate - VAT rate | The VAT or sales tax percentage entered for the invoice. | percent |
Step-by-Step Calculation
Calculate additional work
Multiply out-of-scope billable hours by the agreed hourly rate.
additionalWorkCharge = additionalHours * hourlyRate
Build the pre-discount subtotal
Add the fixed retainer, additional work charge and expenses.
serviceSubtotal = monthlyRetainer + additionalWorkCharge + rechargeableExpenses
Calculate the discount
Find the monetary value of the discount from the subtotal.
discountAmount = serviceSubtotal * discountRate / 100
Find the net invoice total
Deduct the discount before calculating VAT.
netInvoiceTotal = serviceSubtotal - discountAmount
Calculate VAT
Apply the entered VAT rate to the discounted net total.
vatAmount = netInvoiceTotal * vatRate / 100
Calculate the final invoice total
Add VAT to the net invoice total.
invoiceTotal = netInvoiceTotal + vatAmount
Monthly retainer invoice with extra work and discount
Additional work charge
5 × £90.00
£450.00
Subtotal before discount
£1,200.00 + £450.00 + £150.00
£1,800.00
Discount amount
£1,800.00 × 10%
£180.00
Net invoice total
£1,800.00 − £180.00
£1,620.00
VAT amount
£1,620.00 × 20%
£324.00
Total invoice amount
£1,620.00 + £324.00
£1,944.00
Final Result
Estimated monthly invoice total: £1,944.00, including £324.00 VAT.
Assumptions
- ✓All amounts use the same currency.
- ✓One hourly rate applies to all additional work in the month.
- ✓The discount applies to the complete subtotal, including rechargeable expenses.
- ✓VAT is calculated on the discounted net invoice total.
- ✓Amounts are rounded for display after calculation.
Limitations
- !The calculator uses one VAT rate across the entire invoice.
- !It does not separate taxable, exempt, zero-rated or differently treated items.
- !Invoice-level rounding practices may create a small difference from line-by-line calculations.
- !It does not assess whether expenses should be recharged or how they should be treated for tax.
Common Mistakes to Avoid
Entering a VAT-inclusive retainer when the calculator expects a fee before VAT.
Applying the discount after VAT instead of before VAT.
Forgetting to include work completed outside the retainer.
Entering an hourly charge as the number of hours, or vice versa.
Using a single VAT rate where invoice lines need different treatment.
Related Formulas
Frequently Asked Questions
How do you calculate an accountant's monthly invoice total?
Add the pre-VAT retainer, additional-hours charge and rechargeable expenses, subtract the discount, then add VAT to the remaining net amount.
Is VAT calculated before or after a discount?
This calculator calculates VAT after the discount, using the discounted net invoice total.
How are additional billable hours charged?
Additional hours are multiplied by the additional-work hourly rate.
Does the formula include expenses?
Yes. Rechargeable expenses are included in the subtotal before the entered discount and VAT calculation.
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