Enter a loan balance, annual interest rate, and the weekly interest value to calculate the missing variable.
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Weekly Interest Formula
I_week = B * (APR / 52)
- I_week = interest charged for one week
- B = current loan balance
- APR = annual interest rate as a decimal (6% = 0.06)
For multiple weeks with simple interest, multiply by the number of weeks: I = B * APR * (weeks / 52). For weekly compounding, use End = B * (1 + APR/52)^weeks. To back into the APR from a known weekly charge, use APR = (I_week / B) * 52. These formulas assume the balance does not change during the period and ignore fees.
Reference Tables
Weekly interest on a $10,000 balance at common APRs:
| APR | Weekly rate | Weekly interest | Annual interest |
|---|---|---|---|
| 3% | 0.0577% | $5.77 | $300 |
| 6% | 0.1154% | $11.54 | $600 |
| 10% | 0.1923% | $19.23 | $1,000 |
| 18% | 0.3462% | $34.62 | $1,800 |
| 24% | 0.4615% | $46.15 | $2,400 |
| 29.99% | 0.5767% | $57.67 | $2,999 |
How to read your APR:
| Range | Status | Typical product |
|---|---|---|
| Below 6% | Low | Mortgage, secured loan |
| 6% to 18% | Moderate | Auto, personal loan |
| Above 18% | High | Credit card, short-term loan |
Worked Example and FAQ
Example. You owe $4,500 on a credit card at 22.99% APR. Weekly interest is $4,500 × (0.2299 / 52) = $19.89. Holding the balance for 8 weeks accrues about $159.16 in simple interest, or $159.69 with weekly compounding.
Why divide APR by 52? There are 52 weeks in a year, so dividing the annual rate gives the periodic rate for one week. Some lenders use a daily rate (APR / 365) and multiply by 7 instead. The result is nearly identical.
Does this match what my lender charges? Close, but not exact. Most lenders accrue interest daily on the average daily balance, so payments and new charges shift the total. Use this as a quick estimate, not a substitute for your statement.
When should I use compounding? Use weekly compounding if interest is added to the balance each week and then earns more interest. For a single billing cycle or a held balance, simple interest is close enough.
