Daily Interest Formula
Calculate interest per day from principal and an annual rate.
Show the math →Financial math & formulas
See the Math Behind the Calculation
Learn how daily interest, per diem interest, periodic rates, amortized loan payments, and other financial calculations work. Each page defines the formula, works through a numerical example, and links to a calculator where you can check your own numbers.
Start with the formula you need
Financial math library
These pages focus on the arithmetic and rate conventions behind common loan and interest questions.
Daily Periodic Rate Formula
Convert APR or a nominal annual rate into a daily periodic rate.
Show the math →Annual Rate to Daily Rate
See both nominal and effective daily-rate conversions.
Show the math →Annual Rate to Monthly Rate
Understand annual rate ÷ 12 versus an effective monthly equivalent.
Show the math →Daily Rate to Annual Rate
Convert a daily periodic rate back to nominal or effective annual form.
Show the math →Per Diem Interest Formula
Calculate the dollar amount of loan interest that accrues each day.
Show the math →Loan Amortization Formula
Calculate a level loan payment and understand principal and interest.
Show the math →Rule of 78 Formula
Calculate sum-of-digits weights and unearned-interest rebates.
Show the math →How the site fits together
Formula → Calculator → Guide → Software
Use Math when you want the equation, Calculators when you want the answer, Guides when you want the practical explanation, and Windows software when you need editable schedules, saved loans, reports, or ongoing loan management.
Popular starting point
Daily interest and per diem
Google searchers already use Power of Interest for daily-interest and per-diem calculations. These formula pages explain the exact math behind those tools.
Daily interest formula
Principal × annual rate ÷ day-count basis, with 360/365 examples.
Per diem interest formula
Calculate interest per day for loan balances and payoff dates.
Annual rate to daily rate
Learn the difference between nominal division and an equivalent effective rate.