PPF interest is worked out every month on the lowest balance between the 5th and the end of the month, and credited once a year on 31 March. So a deposit made on or before the 5th of April earns for the whole year - the table shows how much depositing in April beats spreading it over the year.
The maturity is tax-free, and deposits up to ₹1,50,000 a year count towards Section 80C under the old tax regime.
How it is worked out
Each year: interest = Σ (balance after deposits by the 5th of the month) × r / 12 over the 12 months, credited at the year end.
Questions
What happens after 15 years?
You can close the account, or extend it in blocks of 5 years, with or without fresh deposits. The extension option here shows both.
When can I take a loan or withdraw?
A loan from the 3rd to the 6th year (up to 25% of the balance two years back). A partial withdrawal once a year from the 7th year (up to 50% of the lower of the balance four years back and last year). The table lists the limit each year.
Which rate should I use?
The government sets the PPF rate every quarter. The default here is the rate most recently known; change it if it has moved.