Daily Compound Interest Calculator

Calculate investment growth with daily compounding and optional monthly contributions.

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Enter your values and click Calculate

Daily compounding means interest is calculated and added to your principal every single day — 365 times per year instead of monthly or annually — giving you slightly more growth because your newest interest starts earning its own return immediately. While the difference between daily and monthly compounding is modest at lower rates (a few dollars on a $10,000 balance over a year), it becomes more meaningful over long time horizons and at higher rates. High-yield savings accounts, money market accounts, and many certificates of deposit (CDs) compound daily, making this the most relevant compounding frequency for cash savings products. This calculator also accepts an optional monthly contribution, which significantly amplifies long-term results through dollar-cost averaging and the compounding of additional capital over time. An investor contributing $200 a month to a daily-compounding account at 7% for 20 years will accumulate dramatically more than a lump-sum deposit without contributions. The calculator runs a day-by-day simulation to handle monthly contributions accurately, then reports future value alongside total interest earned and total contributions — giving you a precise breakdown of how much came from compound growth versus what you deposited. Use it to compare savings accounts, evaluate CD offers, or model how regular contributions accelerate wealth building over multi-year savings goals.

How It Works

The daily rate is calculated as: annual rate ÷ 365. Each day, the balance is multiplied by (1 + daily rate) to apply that day's interest. Monthly contributions are added once every approximately 30.4 days (365 ÷ 12) using a day-by-day simulation loop rather than a closed-form formula — this approach handles the interaction between daily compounding and periodic contributions more accurately. As a worked example: $10,000 at 5% annual rate for 10 years with no contributions. Daily rate = 0.05 ÷ 365 ≈ 0.01370%. After 3,650 days, balance = $10,000 × (1.0001370)^3650 ≈ $10,000 × 1.6487 = $16,487. Total interest earned = $16,487 − $10,000 = $6,487. Total interest is always calculated as future value minus total contributions (initial principal plus all monthly deposits), showing exactly how much compound growth added on top of what was deposited.

Examples

$10,000 Lump Sum at 5%
A one-time $10,000 investment at 5% annual rate with daily compounding for 10 years.
Result: Future value ~$16,487. Total interest ~$6,487.
Long-Term Savings with Contributions
$1,000 starting balance at 7% for 20 years with $200/month added regularly.
Result: Future value ~$108,000. Of that, $49,000 was contributed and ~$59,000 came from compound interest.
High-Yield Savings Account
$5,000 in a savings account at 3.5% annual rate for 5 years, no additional deposits.
Result: Future value ~$5,956. Total interest ~$956 earned purely from daily compounding.

Frequently Asked Questions

How much better is daily vs monthly compounding?
The difference is small but real. At 5% APR, $10,000 grows to ~$16,487 with daily compounding versus ~$16,470 with monthly compounding — a difference of about $17 over 10 years. The gap widens at higher rates and longer timeframes: at 10% over 30 years, daily compounding produces roughly $1,300 more on a $10,000 investment than annual compounding. For most savers, the compounding frequency matters far less than the rate itself.
What does APY mean?
Annual Percentage Yield (APY) is the effective annual rate after accounting for compounding. For 5% APR compounded daily, APY = (1 + 0.05/365)^365 − 1 ≈ 5.127%. Banks advertise APY rather than APR because it reflects the true return you actually receive. When comparing savings accounts, always compare APY — not APR — to determine which account pays more.
Does adding monthly contributions make a big difference?
Yes — regular contributions have a far greater impact than compounding frequency alone. Adding $200/month to a $1,000 starting balance at 7% for 20 years produces roughly $108,000, whereas the same $1,000 with no contributions grows to only about $3,870. The compounding of each new deposit over its remaining time horizon is what drives wealth accumulation, which is why starting contributions early is consistently more powerful than chasing a marginally higher interest rate.

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