π Interest Per Day Calculator
What each day costs or earns.
Interest per day
Estimates for planning only, not financial advice.
Daily interest is the principal times the annual rate divided by the day-count basis: $250,000 at 6.5% accrues $44.52 per day on a 365-day basis, or $1,335.62 over 30 days. Mortgages, margin loans, and late payments all tick by the day, so this number is the atomic unit of borrowing cost.
How to Use This Calculator
Enter the principal and the annual rate, then pick the day-count basis your loan uses. Most consumer loans use 365; many commercial loans use a 360-day bank year, which slightly raises the daily figure.
Enter the principal balance.
Enter the annual interest rate and choose 365 or 360 days.
Read daily interest and the 30-day total. For the full loan picture, our loan calculator builds the amortization.
The Daily Interest Formula
This is simple interest per day: each day's charge is independent, and the balance is assumed constant. Real loans compound or amortize, so the daily figure drifts as principal is paid down. The 360-day basis is a quiet markup: dividing by 360 instead of 365 makes each day cost about 1.4% more, which is why commercial borrowers should always ask which basis applies. Per-diem interest also appears at mortgage closing, where you pay daily interest from the funding date to month-end.
Worked Example
Principal $250,000, rate 6.5%, 365-day basis: 250,000 x 0.065 / 365 = $44.52 per day. Over 30 days that is $1,335.62. Switch to a 360-day basis: 250,000 x 0.065 / 360 = $45.14 per day, about $0.62 more daily, or $18.60 over the month. On a $2M commercial loan at 7%, the basis choice is worth roughly $778 a month, which is why treasury teams negotiate it.
Where Per-Day Math Matters
Three places. Mortgage closings: per-diem interest from funding to month-end is a real closing cost, so closing on the 5th versus the 28th moves cash. Margin accounts: brokers accrue daily, and the meter runs weekends too on most. Late payments: penalties and interest often accrue daily from the due date, so a 10-day delay costs a computable amount, not a vague fee. In each case the daily figure turns an abstract rate into a concrete daily price, which is the unit you actually feel.
My habit: convert every rate you are offered into dollars per day before deciding. A 6.5% mortgage sounds abstract; $44.52 a day on $250,000 sounds like a daily choice, because it is. For savings instead of debt, the compound interest calculator shows the same daily math working in your favor.
Estimates for planning only, not financial advice.
Frequently Asked Questions
How do you calculate interest per day?
Multiply the principal by the annual rate and divide by the day-count basis: P x r / 365 for most consumer loans, or / 360 for many commercial loans. The result is the simple-interest charge for one day.
What is per diem interest on a mortgage?
Daily interest charged from the loan funding date through month-end at closing. It is a standard closing cost: closing earlier in the month means more per-diem days. The daily figure comes from this same formula.
365 vs 360 day count: which is right?
Consumer loans typically use 365 (or actual days); commercial and money-market loans often use 360. The 360 basis raises daily interest by about 1.4%. Check the loan agreement rather than assuming.
Does daily interest compound?
This formula gives simple daily interest. Most loans effectively compound through amortization schedules, so the daily charge falls as principal is repaid. Credit cards compound daily on the average balance.
How much is 6% on $100,000 per day?
About $16.44 on a 365-day basis (100,000 x 0.06 / 365). Scale linearly: double the principal or the rate and the daily figure doubles.
Do weekends count for daily interest?
Usually yes for accrual: interest accrues every calendar day on most loans, including weekends and holidays. The day-count basis already reflects this convention.
Why do lenders quote annual rates instead of daily?
Regulation and tradition: APR and APY standardize comparison. But converting to dollars per day, as this calculator does, makes the cost tangible before you sign.
Can daily interest change during the loan?
The daily figure changes when the principal changes (payments) or when a variable rate resets. Fixed-rate amortizing loans see daily interest fall steadily as principal shrinks.
What is the difference between APR and daily interest?
APR is the annualized cost disclosure; daily interest is the same cost expressed per day. Convert between them with this calculator to feel what the APR really charges every morning.
How is per-diem interest calculated at closing?
Multiply the loan amount by the note rate, divide by 365, and multiply by the days from funding through month-end. It is itemized on the closing disclosure, so verify it against this math.
Does making extra payments change daily interest?
Yes, immediately. Each extra principal payment lowers the balance the daily formula multiplies, so the next day's interest drops. That is why early payments beat late ones even within the same month.