Savings Plan Calculator with Bonus (Compound, Taxed Each Period)
A savings simulator that combines regular monthly contributions with lump-sum bonus contributions to estimate your maturity payout. Using a taxed-each-period compound model where tax is deducted every time interest accrues, it shows the after-tax payout, principal, and interest breakdown. The tax rate defaults to Japan's 20.315% flat withholding tax and can be changed freely.
Last updated:
Input
Default is Japan's 20.315% flat withholding tax (15% income tax + 0.315% special reconstruction tax + 5% resident tax)
Result
Maturity amount (after tax)
5,823,568yen
Total principal
5,600,000 yen
Interest after tax
223,568 yen
Maturity amount
5,823,568 yen
Breakdown (monthly / bonus contributions)
Monthly principal
3,600,000 yen
Monthly maturity value
3,746,026 yen
Bonus principal
2,000,000 yen
Bonus maturity value
2,077,541 yen
How it works
- The monthly contribution is treated as an ordinary annuity paid at the end of each period. The monthly rate is (annual rate / 12) multiplied by the after-tax factor (1 - tax rate) to give the after-tax effective rate r'; future value is FV = monthly contribution x ((1+r')^n - 1) / r' (n = contribution years x 12).
- The bonus contribution is handled the same way: the rate per period is (annual rate / number of bonus payments per year), multiplied by (1 - tax rate) to give the effective rate rb', so FV = bonus contribution x ((1+rb')^nb - 1) / rb' (nb = contribution years x bonus payments per year).
- The after-tax maturity amount is the sum of the future value of the monthly contributions and the future value of the bonus contributions. When the rate is 0%, no interest accrues, so each stream's future value is simply the contribution amount x the number of payments.
- Total principal is the sum of the monthly contribution x number of payments (years x 12) and the bonus contribution x number of payments (years x bonus payments per year). After-tax interest is calculated as (maturity amount - total principal).
- This tool uses a taxed-each-period model in which tax is deducted every time interest accrues; by folding the tax rate into the effective rate, it simulates compound growth on an after-tax, take-home basis. The default tax rate is Japan's 20.315% flat withholding tax (15% income tax + 0.315% special reconstruction tax + 5% resident tax), and it can be set to 0% to assume a tax-free account such as NISA.
- Note: with real financial products, the timing of interest accrual, the taxation method, and rounding differ by product. The results here are simplified estimates based on the stated assumptions and do not guarantee your future payout.
Reviews
Tell us what you think of this calculator.
Write a review
Rating