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Constant Balance Daily Interest

Estimate simple daily interest for a fixed balance over a billing cycle or selected date interval.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Results are estimates for planning and education. Confirm rates, taxes, fees, and legal requirements with the relevant institution or a qualified professional before making a financial decision.

Formula and method

A constant balance of 3,650 at 10% APR accrues 1 per day and 30 over a 30-day cycle.

Daily charge = balance × APR/365; finance charge = daily charge × days. Date mode counts both endpoints.

Worked example

Enter these known values and leave the other values blank.

Current balance owed
3650 USD
Annual percentage rate (APR)
10 %
Billing cycle length
30 days
Result
Cycle charge and balance after this simple charge: 30; 3680;

Assumptions and limitations

  • Assume a constant balance and simple daily interest with a 365-day denominator. Transactions, minimum payments, compounding and card-specific methods are omitted.
  • Date mode includes both endpoints; same-day start and end count as one day.
  • Use one currency throughout. Retained currency selector factors are a historical snapshot, not live exchange rates.
  • Clear an editable unknown before solving an inverse. Selection controls and forced outputs cannot be inferred.

Common questions

What does this model assume?

Assume a constant balance and simple daily interest with a 365-day denominator. Transactions, minimum payments, compounding and card-specific methods are omitted.

Can I use the currency menu as a current quote?

No. Keep amounts in one currency; conversion factors are a captured historical snapshot.

References

The calculation follows this reviewed mathematical model: Daily charge = balance × APR/365; finance charge = daily charge × days. Date mode counts both endpoints. Required model inputs: Balance must be nonnegative. APR must be nonnegative.

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