Byaj kaise nikale: how to calculate interest for months and days

The byaj formula

Simple interest (sadharan byaj) needs three things: the amount given (principal), the rate, and the time. With a sekda rate, which is per ₹100 per month, measure the time in months.

If the rate is a yearly percent, the same formula works with months ÷ 12:

10% byaj on ₹1,000: a month or a year?

It depends on who says it. In school maths and at a bank, 10% means a year: ₹1,000 earns ₹100 a year, about ₹8.33 a month. In informal lending a bare "percent" usually means a month: ₹100 every month, ₹1,200 a year. Ask which, and write it down.

Byaj for 1 month, 6 months and 1 year

Work out one month first, then multiply. All three use ₹10,000 at ₹2 sekda.

Byaj for 1 month

10,000 × 2 ÷ 100 = ₹200.

Byaj for 6 months

10,000 × 2 × 6 ÷ 100 = ₹1,200.

Byaj for 1 year

10,000 × 2 × 12 ÷ 100 = ₹2,400. With a yearly rate it is one step: 10,000 × 24 ÷ 100 = ₹2,400 at 24% a year.

Byaj for a few days

Amount × sekda × days ÷ 3,000, because a day is 1/30 of a month: 10,000 × 2 × 10 ÷ 3,000 = ₹66.67. The interest per day calculator does any number of days, and shows a bank's ÷ 365 next to it.

With dates that run past a month, the only tricky part is the time. Here is how a bahi khata counts it.

How to count months and days

1. Count whole months by date. Start from the date the money was given and step forward a calendar month at a time. From 15 Jan, one month later is 15 Feb. One more would be 15 Mar, which is past 10 Mar, so there is 1 whole month.
2. Count the leftover days. From 15 Feb to 10 Mar is 23 days.
3. Put them together. 15 Jan to 10 Mar is 1 month 23 days, which is 1 + 23/30 = 1.7667 months.

A whole month is a whole month, whether it has 28, 30 or 31 days: 15 Jan to 15 Feb counts as one month. Only the leftover days are turned into a fraction, at 1/30 of a month each. That is why the bahi count can differ a little from the calendar: this period is 54 actual days.

Worked example: ₹2 sekda for 1 month 14 days

You lent ₹50,000 at ₹2 sekda on 5 Mar 2026, and it comes back on 19 Apr 2026.

1. Whole months: 5 Mar → 5 Apr = 1 month.
2. Leftover days: 5 Apr → 19 Apr = 14 days.
3. Time: 1 + 14/30 = 1.4667 months.
4. Interest for one month: ₹50,000 × 2 ÷ 100 = ₹1,000.
5. Interest: ₹1,000 for the month + ₹466.67 for 14 days = ₹1,466.67.
6. Total to return: ₹50,000 + ₹1,466.67 = ₹51,466.67.

Worked example: a yearly percent rate

₹2,00,000 at 18% a year, from 10 Jan 2026 to 25 Jun 2026. The time is 5 months 15 days, or 5.5 months.

• Using the yearly rate: ₹2,00,000 × 18 ÷ 100 × 5.5 ÷ 12 = ₹16,500.
• Using sekda: 18% a year ÷ 12 = ₹1.5 sekda, so ₹3,000 a month × 5.5 = ₹16,500.

Both roads give the same answer. Use whichever is easier for your rate. What rupaye sekda means has a full table for switching between the two.

How to calculate byaj on a mobile calculator

Any phone's calculator will do. Use only ×, ÷ and =; skip the % key, which works differently on different phones. Type the numbers without commas.

1. Whole months: amount × sekda × months ÷ 100 =. For ₹50,000 at ₹2 sekda for 6 months, press 50000 × 2 × 6 ÷ 100 = and read 6000.
2. Months and days: turn the days into a part of a month first (days ÷ 30), then use it as the months. 1 month 15 days is 1.5, so 50000 × 2 × 1.5 ÷ 100 = 1500.
3. Only days: amount × sekda × days ÷ 3000 =. 10 days: 50000 × 2 × 10 ÷ 3000 = 333.33 (the screen shows more decimals; round to paise at the end).
4. A yearly % rate: amount × rate × months ÷ 1200 =. ₹2,00,000 at 18% a year for 3 months: 200000 × 18 × 3 ÷ 1200 = 9000.

Loans given on the 29th, 30th or 31st

Not every month has a 31st. When money is given on 31 Jan, the common rule, and the one our calculator follows, is that its month falls on the last day of a shorter month. So 31 Jan to 28 Feb is 1 month (29 Feb in a leap year). Later months fall back on the 31st where the month has one.

Months and days vs actual days ÷ 365

Banks usually count actual days over a 365-day year. At the same rate (₹2 sekda is 24% a year), here is how the two methods compare on ₹50,000:

• Across February the bahi method charges ₹67.12 more. February's 28 days count as a full month, and a bank's day is worth a little less than a bahi day (a year ÷ 365 rather than a month ÷ 30).
• Across a 31-day month actual days charge ₹12.78 more, because the 31st day counts too.
• Over a whole year both give ₹12,000. The gap only comes from short and long months inside the period.

Neither method is wrong. What matters is that both sides use the one they agreed. Loans quoted in sekda are usually worked out the bahi way; a yearly rate from a bank or finance company usually uses actual days. Our calculator does both.

Common mistakes in byaj calculation

• Reading sekda as a yearly rate. ₹2 sekda is 24% a year, not 2%. Read as 2% a year, the first example would give ₹122.22 instead of ₹1,466.67.
• Mixing the two methods by accident. Counting actual days and dividing by 30 is a third method, neither the bahi way nor the bank's; use it only if that is what was agreed. For 10 Jan to 25 Mar it gives ₹2,466.67, against ₹2,500 by the bahi.
• Charging interest on unpaid interest. Under simple interest, interest is only on the principal. ₹50,000 at ₹2 sekda earns ₹12,000 a year, so ₹24,000 over two years, even if nothing was paid in the first year. Adding unpaid interest to the principal is compound interest, and needs to be agreed: see simple vs compound interest.
• Ignoring part payments. If part of the money came back along the way, work out the interest up to each payment first. Part payments: interest first or principal first shows how.