What this calculator does
This tool inverts the simple-interest formula to answer a single question: how many days does it take for a principal (PV) to grow into a target final amount (FV) when interest does not compound? Simple interest accrues only on the original principal, so growth is strictly linear over time. The math is universal, but the "Rate Type" selector also supports a per-day rate convention (called "hibu" in Japan) that expresses the rate as a daily percentage.
How to use it
Pick a Rate Type to set the time basis (mode): an annual rate uses a 365- or 360-day year, a monthly rate uses 30 days per month, and a daily rate applies per single day. Enter the Interest Rate as a percent, the Principal you start with, and the Final Amount you want to reach (principal plus accrued interest). The calculator returns the number of elapsed days required.
The formula explained
Simple interest gives FV = PV × (1 + r_{frac} × days / mode), where r_{frac} = rate / 100. Solving for days yields:
The "mode" constant is the number of days in one full rate period: 365 or 360 for annual rates, 30 for monthly, and 1 for daily. Because the rate is divided by 100 internally, you enter 5 for 5%, not 0.05.
Worked example
Start with PV = 100,000 and target FV = 120,000 at a 5% annual rate on a 365-day basis. Then r_{frac} = 0.05, and FV/PV - 1 = 0.2. So
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輸入各項數值時請務必確認量綱（例如小時、天數、金額、百分比），避免單位不匹配引發推算偏差。
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建議針對極端值與多組預期情景分別試算，觀察輸出數據的波動區間，輔助做出更穩健的決策。
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本工具運算結果供日常排程、學業研習與事前參考之用，如涉及重大法規或投資決策請諮詢持證專家。