Regular Monthly Deductions and Arrears: Calculating Interest on Wage Claims and Underpayment
The Problem
When an employer underpays wages, commission, or other regular payments over an extended period, interest does not accrue from the end of employment or from the date of the tribunal judgment. It accrues from each date a payment was due and not made. On a two-year underpayment claim, that can mean 24 separate interest calculations, each with a different accrual period.
Most schedules of loss get this wrong. And while individual deductions may seem modest when calculated piecemeal, the cumulative interest on a genuine underpayment can be substantial—and to the individual concerned, that money matters.
The Principle
An employee (or other regular payee) who suffers repeated shortfalls is entitled to interest on each shortfall from the date it should have been paid. The first missed or short payment accrues interest for longer than the last. If you calculate interest only from the date of termination, or from the date the ET1 was filed, you are systematically understating the claimant's loss.
Worked Example: Wage Underpayment
An employee should have been paid £2,000 per month (gross) but was actually paid only £1,600 per month, for 24 months (January 2023 through December 2024). The tribunal enters judgment in June 2025. Statutory interest is 8% per annum, simple interest.
The principal loss is £400 × 24 = £9,600. But the interest is not £9,600 × 8% × (some average time). It is calculated month by month:
January 2023 shortfall (£400): Due 31 Jan 2023. Interest runs from 31 Jan 2023 to June 2025 = ~29 months = ~880 days.
Interest = £400 × 8% × (880 ÷ 365) = £77.26
February 2023 shortfall (£400): Due 28 Feb 2023. Interest runs for ~850 days.
Interest = £400 × 8% × (850 ÷ 365) = £74.52
March 2023 shortfall (£400): Due 31 Mar 2023. Interest runs for ~820 days.
Interest = £400 × 8% × (820 ÷ 365) = £71.78
…and so on, month by month, until:
December 2024 shortfall (£400): Due 31 Dec 2024. Interest runs for ~180 days.
Interest = £400 × 8% × (180 ÷ 365) = £15.78
Summing all 24 months of interest comes to approximately £1,100 (depending on exact day-count). That is more than 11% of the principal.
For the employee, £1,100 is real money. It is the difference between a subsistence settlement and one that covers genuine loss. For the employer or their insurance company, it is a material underestimate in settlement discussions if they ignore it.
Why the Individual Calculations Matter
Each month's shortfall sits unpaid for a different length of time:
- The first shortfall (Jan 2023) is unpaid for 29 months by the time judgment is entered.
- The last shortfall (Dec 2024) is unpaid for only 6 months.
- The difference in accrued interest between these two periods is significant (£77 vs £16—nearly 5x).
If you used a simplistic "average" period (say, 17.5 months for all shortfalls), you would understate interest on the early shortfalls and overstate it on the later ones. On a 24-month underpayment, the errors cancel out somewhat—but on a 36-month claim, or on a higher rate, they compound badly.
Scenario 2: Commission Underpayment
A salesperson should have earned £1,000/month in commission but was paid only £600/month for 18 months. The shortfall is only discovered or litigated later. Each £400 shortfall accrues interest from the month-end when it was due.
By the time of settlement or judgment, the first shortfall might have been outstanding for two years; the last for only six months. The interest on the first is materially higher than on the last. Averaging them, or applying a single "global" interest rate, understates the claimant's actual loss.
Jurisdiction-Specific Notes
United Kingdom
Employment tribunals have discretion to award interest under the Employment Tribunals (Interest on Awards in Discrimination Cases) Regulations (for discrimination claims) or under general principles for contract/wages claims. The rate is often the judgment debt rate (currently 8%) or a rate the tribunal considers just. The principle—interest from the date each sum was due—is the same.
United States
State employment statutes vary. Some states mandate prejudgment interest on wage claims; others leave it to the court's discretion. Where interest is available, it typically runs from the date each wage payment was due, not from the date of suit. Day-count and compounding rules vary by state, but the segmentation principle holds.
Canada
Provincial employment standards and wage recovery statutes have similar provisions. Interest on wage arrears typically accrues from the date each payment was due. Consult your provincial statute for the applicable rate and whether interest is mandatory or discretionary.
How to Calculate Correctly
Create a table: one row per month (or pay period).
| Month | Date Due | Shortfall | Days Outstanding | Interest @8% |
|---|---|---|---|---|
| Jan 2023 | 31 Jan | £400 | 880 | £77.26 |
| Feb 2023 | 28 Feb | £400 | 850 | £74.52 |
| Mar 2023 | 31 Mar | £400 | 820 | £71.78 |
| … | … | … | … | … |
| Dec 2024 | 31 Dec | £400 | 180 | £15.78 |
| Total | £9,600 | ~£1,100 |
Sum column 4 (or use the formula row by row). That is your interest claim.
For Claimants and Their Advisors
Individual underpayments can feel modest month to month. But over 18–36 months, with compounding interest, they become substantial. A £400/month shortfall for 24 months is £9,600 in principal—but with proper interest calculation, it becomes £10,700 or more. That is a 10%+ uplift.
Do not leave that money on the table because the calculation is tedious. Show your working. Most respondents will not challenge properly calculated interest; many will not have calculated it themselves and will accept your figure if it is clearly supported.
For Respondents and Their Counsel
If you're defending a wage claim, calculate interest early and accurately. It gives you a realistic view of your exposure and avoids nasty surprises when the claimant's solicitor produces a detailed schedule at the remedies hearing.
If the claimant's schedule of loss undercalculates interest, do not assume they will forget to update it. Good advisors catch this and recalculate at the last moment. Build the proper interest liability into your settlement authority and reserve.
The Practical Value of Automation
For practitioners handling multiple wage claims, employment disputes, or underpayment cases, manually calculating interest month by month is error-prone and time-consuming. A tool that breaks down interest by pay period, applies the correct rate for each period, and generates a transparent schedule automatically removes that friction.
The result: more accurate claims, faster settlements, fewer disputes over whether the interest was calculated right. For your clients—especially individuals for whom that interest represents real money—it means they get paid what they are legally owed.
Calculate Wage Arrears Interest
Use the recurring principal feature to calculate interest on monthly shortfalls, each accruing from its due date.