Why does the date of my deposit matter?

Because interest for a month is calculated on the lowest balance between the close of the 5th and the end of the month. Money that arrives on the 6th is not part of that lowest balance, so it earns nothing until the following month. Once a year, that is a month of interest on the whole deposit.

Is a lump sum in April better than monthly instalments?

At the same annual total, the lump sum is in the account for more months, so more interest accrues on it. Whether you can spare the whole amount in April is a separate question, and the calculator lets you compare the two directly.

Why does this give a different figure from other PPF calculators?

Many of them compound a yearly deposit annually, which ignores both the monthly accrual and the fifth-of-the-month rule. This follows the scheme text, so it will differ from any model that does not — and it will differ from your passbook too if your deposit dates differ from the schedule you chose here.

Is PPF interest taxable?

The scheme states that interest earned in the account is exempt under section 10 of the Income Tax Act, and deposits qualify for deduction under section 80C. Tax law changes, so check the position for the year you are in.

What rate should I put in?

Whichever you want to test. The rate is notified by the government rather than fixed by the scheme, so this calculator does not assume one is current — try a range and see how much the answer moves.

PPF Calculator

The Public Provident Fund credits interest once a year, but accrues it monthly on the lowest balance between the close of the 5th and the end of the month. This projects a balance under that rule rather than compounding a yearly figure, so the answer reflects when you deposit as well as how much.

Check the working

A worked example

A fixed case, for reference.

₹1,50,000 deposited at the start of each year, at an assumed 7.1%, held for the full fifteen-year term.

Year one, deposited by the 5th of the first month
$150000 \times 7.1\% = 10{,}650$
The same deposit made after the 5th earns for eleven months
$150000 \times \frac{0.071}{12} \times 11 = 9{,}762.50$
Over fifteen years, deposits of
$150000 \times 15 = 22{,}50{,}000$
Balance at maturity
$\approx 40{,}68{,}209$

Interest credited comes to about ₹18,18,000 on ₹22,50,000 deposited. Missing the 5th every year would leave roughly ₹22,475 of that on the table, and spreading the same money across twelve instalments instead of one April deposit would cost about ₹1,23,610.

The formula

I_m = \min(B_m) \times \frac{r}{12}

Interest accrues for each calendar month on the lowest balance in that month, between the close of the fifth day and the end of the month.

B_{\text{year end}} = B + \sum_{m=1}^{12} I_m

The twelve monthly accruals are added to the account once, at the end of the year. Nothing compounds within the year.

What each symbol means

B: the balance in the account
min(Bₘ): the lowest balance in month m, after the 5th
r: the annual rate you are assuming, as a decimal
Iₘ: interest accrued for month m, not yet credited

What this assumes, and where it stops

Assumptions

The rate you enter applies unchanged for the whole term. The government sets the PPF rate and has revised it repeatedly, so a single rate across fifteen years is a simplification.
Deposits are made on the same schedule every year, with no missed year and no default fee.
The deposit stays within the scheme limits of ₹500 to ₹1,50,000 in a financial year, counted across your own accounts and any opened for a minor.
Interest accrues monthly on the lowest balance between the close of the 5th and the end of the month, and is credited at the end of the year — paragraph 7 of the Public Provident Fund Scheme, 2019.

Limitations

Loans against the balance, permitted between the third and sixth years, are not modelled.
Partial withdrawals, permitted from the seventh year, are not modelled — a withdrawal lowers the balance interest is calculated on.
Premature closure is not modelled. The scheme allows it after five years but reduces the interest rate by one percentage point across the account’s whole life.
The year is treated as twelve equal months beginning with your first deposit, rather than as specific calendar dates.
Extensions are modelled as continuing deposits at the same rate. An extension can also be taken without further deposits, which this does not cover.

What this calculator does

Projects the balance at maturity, or at the end of an extension block, from the deposit and rate you choose.
Applies the scheme’s own accrual: monthly, on the lowest balance between the 5th and month-end, credited at the end of each year.
Shows what depositing on or before the 5th adds against depositing after it.
Shows what a single deposit early in the year adds against twelve monthly instalments of the same total.

Common questions

Different questions about the same money. These use the same conventions, so the numbers are comparable.

FD Calculator
Work out what a fixed deposit pays at maturity, and see how much more you get by reinvesting the interest instead of having it paid out.
RD Calculator
Calculate the maturity value of a recurring deposit, compounded quarterly the way banks do it, and see why the interest is smaller than it sounds.
Investment Goal Calculator
Start from the amount you want and work backwards to the monthly investment it would take to get there.
Inflation Calculator
Find what a sum today is worth in future purchasing power, and what a future target costs in today’s money.