Simple interest calculator

Calculate simple interest (I = P × r × t) and the maturity amount on a principal for a term in years, months or days.

更新日 2026年9月28日検証済みの例：5

利息

小数点以下の桁数：2；最も近い値へ、等距離なら末尾が偶数の値へ

Total amount: $11,800.00
Interest per year: $600.00
Amount if compounded yearly: $11,910.16

At 6% simple interest, $10,000.00 earns $600.00 a year, so $1,800.00 over the term. Compounding yearly at the same rate would give $110.16 more.

Amount at maturity

元金84.7%利息15.3%

Balance over time

Simple interestCompounded yearly

Interest by year 行数：3

Period	Interest in period	Interest so far	残高
Year 1	$600.00	$600.00	$10,600.00
Year 2	$600.00	$1,200.00	$11,200.00
Year 3	$600.00	$1,800.00	$11,800.00

計算方法 S

Time in years
$t = 3 = 3$
利息
$I = P r t = 10{,}000.00 \times 0.06 \times 3 = 1{,}800.00$
Total amount
$A = P + I = 10{,}000.00 + 1{,}800.00 = 11{,}800.00$
Rounded half-to-even to 2 decimals for display only.

Simple interest calculatorについて

Simple interest is paid only on the original principal: I = P × r × t, where r is the yearly rate as a decimal and t the time in years. A term in months is divided by 12, and a term in days by 365 (actual/365) or by 360, the banker's year some lenders and money markets use. The amount due at maturity is A = P(1 + rt).

With the defaults, 10,000 at 6% for 3 years earns 600 a year and 1,800 in total, for a maturity amount of 11,800. Compounded yearly at the same rate, the same deposit would reach 11,910.16, which is 110.16 more.

Interest here never earns interest of its own. Savings accounts and most deposits compound, so use the compound interest calculator for them; simple interest suits short loans, flat-rate quotes and interest accrued between coupon dates.

計算例

10,000 at 6% for 3 years

元金: 10,000
Interest rate (per year): 6%
時間: 3
Time in: 年

利息: 1,800.00
Total amount: 11,800.00
Amount if compounded yearly: 11,910.16

照合元：10000 × 0.06 × 3 by hand; 10000 × 1.06³ = 11910.16 (Python decimal)

5,000 at 8% for 9 months

元金: 5000
Interest rate (per year): 8%
時間: 9
Time in: か月

利息: 300.00
Total amount: 5,300.00

照合元：5000 × 0.08 × 9/12 by hand

20,000 at 7.5% for 90 days, actual/365

元金: 20,000
Interest rate (per year): 7.5%
時間: 90
Time in: Days
Days in a year: 365 (actual/365)

利息: 369.86

照合元：20000 × 0.075 × 90/365 = 369.8630… (Python decimal)

Same loan on a 360-day year

元金: 20,000
Interest rate (per year): 7.5%
時間: 90
Time in: Days
Days in a year: 360 (banker's year)

利息: 375.00

照合元：20000 × 0.075 × 90/360 by hand

よくある質問

How do you calculate simple interest?

Multiply the principal by the yearly rate and by the time in years: I = P × r × t. 10,000 at 6% for 3 years earns 10,000 × 0.06 × 3 = 1,800, so 11,800 is repaid. For a term in months, divide by 12 first: 5,000 at 8% for 9 months earns 5,000 × 0.08 × 9/12 = 300.

How do you calculate simple interest for a number of days?

Divide the days by the day-count basis before multiplying. 20,000 at 7.5% for 90 days earns 20,000 × 0.075 × 90/365 = 369.86 on an actual/365 basis, and 375.00 on a 360-day banker's year. Excel's ACCRINTM function uses basis 3 for actual/365 and basis 2 for actual/360.

How do you find the rate or the time from simple interest?

Rearrange I = P × r × t. The rate is r = I ÷ (P × t): earning 1,800 on 10,000 over 3 years means 1,800 ÷ 30,000 = 6% a year. The time is t = I ÷ (P × r): earning 3,000 on 10,000 at 6% takes 3,000 ÷ 600 = 5 years.

Is simple interest better than compound interest?

For a borrower, yes; for a saver, no. Simple interest grows in a straight line, while compound interest also earns interest on interest. On 10,000 at 6%, the gap is only 110.16 after 3 years (1,800 against 1,910.16), but after 20 years simple interest totals 12,000 while yearly compounding adds 22,071.35.

「Simple interest calculator」の精度はどのくらいですか？

精度は入力値と計算方法の前提に依存します。十進演算には有効数字50桁を使いますが、推定、数値計算手法、元データの精度はそれより低い場合があります。表示の丸め処理でこれらの制約がなくなるわけではありません。独立した出典の解答と照合した計算例：5。例えば、「10,000 at 6% for 3 years」は10000 × 0.06 × 3 by hand; 10000 × 1.06³ = 11910.16 (Python decimal)と照合しています。

この計算方法の出典は何ですか？

OpenStax Prealgebra 2e — Simple Interest Applications (I = Prt); Brealey, Myers & Allen — Principles of Corporate Finance, ch. 2 (simple vs compound interest); Microsoft Excel ACCRINTM function (simple interest accrued to maturity, basis 2 = actual/360, 3 = actual/365).

この計算機について

I = P \cdot r \cdot t,\qquad A = P(1 + r t)

出典

OpenStax Prealgebra 2e — Simple Interest Applications (I = Prt)
Brealey, Myers & Allen — Principles of Corporate Finance, ch. 2 (simple vs compound interest)
Microsoft Excel ACCRINTM function (simple interest accrued to maturity, basis 2 = actual/360, 3 = actual/365)

計画の参考用です。金融機関、税務当局、市場は独自の丸め方、手数料、規則を適用します。契約などの前に該当機関へ金額を確認してください。

出典と照合済み

この計算機には、独立した出典の解答を使った計算例が 5 件あります。テストに組み込まれており、ここでも実行できます。