Explain why the timing of a top-up changes the interest it earns.
Check the date of your last CPF cash top-up, or the date you plan to make this year's, and note which month it falls in. Timing does not change whether a top-up is a good idea. It changes how much the top-up earns in its first year, and that first-year difference repeats for every year you top up.
CPF calculates interest each month on the lowest monthly balance, the lowest amount in your account during that month. Money you add during a month does not raise that month's lowest balance, so it starts earning from the following month. Lesson 1.2, "How CPF interest is calculated and credited", sets out this rule in full.
The month your money lands therefore decides how many months it earns in the first year. A top-up landing in January earns interest from February to December, which is 11 months. One landing in June earns for 6 months. One landing in December earns nothing for that year and starts earning in January.
To work out first-year interest, multiply the amount by the annual rate, then by the number of months earning, and divide by 12. Months earning are the months remaining after the month the money lands.
Linda is 52. She keeps her CPF top-up money in a fixed deposit all year and transfers it to her Special Account (SA) on the last working day of December, every year. She says she does this for the tax relief. Her daughter works out that the same money moved in January would have earned most of a year's extra CPF interest, with exactly the same tax relief.
Take Linda's top-up as S$5,000 and the interest rate as 5% a year. Both are example figures, and 5% is not the current CPF rate. A January top-up earns S$5,000 x 5% x 11 / 12 = about S$229.17 in its first year. A December top-up earns nothing in that year.
After the first year, the two top-ups earn the same rate on the same amount. The January one stays ahead by its first-year interest, and that interest also compounds. After ten years, the January top-up grows to about S$8,112.15 and the December one to about S$7,756.64, a gap of about S$355.51. The gap on one top-up is not life-changing, but repeated every year for twenty years it adds up to a meaningful amount. The only cost is moving the date.
Tax relief is the usual reason people give for waiting until December, and it rests on a misunderstanding. Relief is given for top-ups made in a calendar year, as long as the money arrives by 31 December. A January top-up and a December top-up in the same year get the same relief, claimed in the same tax assessment the following year. Waiting until December adds no relief and only loses interest.
There are legitimate reasons to wait. The cash may not be available until a bonus arrives. You may want to keep the money in your buffer, the cash you keep aside for emergencies, until the year is nearly over and nothing has gone wrong. Or you may want to see your final income for the year before deciding how much relief you can use, since reliefs above the yearly cap save nothing, as Lesson 3.2, "Cash top-ups for yourself and your family", explains. Each of these is a real trade-off between interest and cash flow.
Linda keeps a December date in the end, because her fixed deposit pays her some interest in the meantime and she likes being sure her buffer is untouched. She now moves the money in the third week of December instead of on the 31st.
Almost everyone gets the turn of the year wrong. A top-up on the last working day of December beats one in the first week of January on both interest and relief. The late-December top-up qualifies for that year's relief and, because it is in the account on 1 January, earns interest from January onwards. The early-January top-up earns only from February, and its relief counts for the following year.
If you are topping up around the turn of the year anyway, choose late December over early January. Leave a few days for the payment to be processed, and check the CPF Board's guidance on cut-off dates for your payment method.
None of the three usual patterns is wrong. Each suits a different situation:
Whole amount in January: only if the cash is ready and your buffer stays intact without it. This gives the most interest. Monthly, for example a twelfth of the yearly amount each month: this keeps your cash flow even and earns roughly half of what a January lump sum would. On Linda's S$5,000, first-year interest is about S$114.58, against S$229.17 for January and nothing for December. Once in late December, when your income and bonus for the year are known: the least interest in year one, and the most certainty.
Decide whether interest or cash flow matters more to you this year, then pick a month. Before you do, put a number on the trade by working out what a top-up made in January would earn compared with the same top-up in December, using the current SA rate from cpf.gov.sg.
Using the current SA rate, work out how much more interest a January top-up earns than the same top-up made in December.
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