Tax Refund Foregone Interest Calculator
Cost of over-withholding tax all year.
Calculate the interest you give up by over-withholding tax and getting a refund instead of accurate withholding. Free — no signup.
What this tool does
A tax refund means you lent the government money interest-free. Enter the refund amount, the months it was over-withheld, and a savings interest rate. The tool shows the interest you could have earned.
Enter Values
Formula Used
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Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
A 2,400 refund for over-withholding all year at a 5% savings rate cost about 55 in foregone interest — small per refund, but a habitual over-withholder loses meaningfully over a working life. Adjusting withholdings or estimated payments to be more accurate keeps the cash productive in your account.
What the result means
Foregone interest is what the cash would have earned if held in your savings instead of with the tax authority. Adjusting your withholdings or estimated payments captures the gain.
A worked example
Try the defaults: refund amount of 2,400, months over-withheld of 6, savings rate of 5%. The tool returns 60.00. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.
What moves the number most
The result responds to Refund Amount, Months Over-Withheld, and Savings Rate. Not every input has equal weight. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.
The formula behind this
Foregone interest is refund times annual rate times the fraction of a year the cash was held. Compounding effects are ignored at this scale. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.
Using this in pay negotiations
Knowing the exact figure behind a headline rate gives you specific numbers to anchor to in conversations about pay. "The difference is £X per month after tax" lands harder than "a couple of grand a year". Concrete numbers move decisions.
What this doesn't capture
Tax bands, pension contributions, student-loan deductions, and benefits-in-kind sit outside this calculation. The figure is the headline; your actual position depends on local tax rules and personal circumstances. Pair with a dedicated take-home calculator for the full picture.
Foregone interest from this refund is the figure shown above.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
Foregone interest is refund times annual rate times the fraction of a year the cash was held. Compounding effects are ignored at this scale.
References
Frequently Asked Questions
Is a refund really bad?
How do I adjust withholdings?
Forced savings argument?
What if rates are zero?
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