A year contains 12 months, 26 fortnights and 52 weeks. Those three numbers do not divide into each other cleanly, and almost every frequency mistake in personal finance comes from assuming they do.
Fortnightly is not half of monthly
If monthly rent is $2,000, the fortnightly equivalent is not $1,000. Twenty-six payments of $1,000 collects $26,000 a year against $24,000 for monthly. The tenant would be paying an extra month's rent without anyone intending it.
The correct conversion goes via the annual total: $24,000 a year over 26 fortnights is $923.08. The habit worth forming is to annualise first and convert second, never to halve or double directly.
The same increase costs different amounts
This is where it bites hardest. A $100 increase per payment sounds like one thing regardless of frequency, and it is three quite different things.
- Monthly: 12 payments, $1,200 a year.
- Fortnightly: 26 payments, $2,600 a year.
- Weekly: 52 payments, $5,200 a year.
The per-payment figure is identical in all three. The annual cost varies by a factor of more than four. The rent increase calculator reports the annual figure alongside the per-payment one because the per-payment number consistently understates what is happening.
Percentages are frequency-neutral, amounts are not
A 5 per cent increase is 5 per cent whether rent is weekly or monthly, because it scales with the base. A fixed dollar increase does not: the same $100 is a larger share of a weekly rent than a monthly one.
So when a notice arrives quoting a dollar figure, converting it to a percentage tells you how it compares with inflation or with the market. When it quotes a percentage, converting to an annual dollar figure tells you what it costs. Both conversions are worth doing, and they answer different questions.
Advertised rents are not comparable across frequencies
Rental listings quote weekly in some markets and monthly in others, sometimes within the same city. A property at $500 a week is $26,000 a year. A property at $2,100 a month is $25,200. The weekly one looks cheaper per payment and costs $800 more.
The only safe comparison is annual. It takes one multiplication and it removes the entire class of error.
The same trap in reverse: the extra payday
Paid fortnightly, you receive 26 payments a year, which is two more than twice-monthly would give. Budgeting as though fortnightly pay is monthly pay divided by two understates annual income by about 8 per cent.
Some people treat those two extra paydays as a windfall, which works. Others find their monthly budget never quite balances, because it was built on a conversion that was wrong from the start. The arithmetic is the same either way, and the pro rata rent calculator handles the related case of a part-period charge.
For the bigger housing decision, see the rent or buy comparison. For a related way that a quoted rate is not the rate you pay, see the piece on compounding frequency.