Compound Interest Calculator for Small Business

Small-business owners care about compound interest differently than retail savers: the interest is usually on a business reserve account or a retained-earnings pool, and the cadence (daily vs monthly) matters more than the headline APR. This calculator is built around that workflow.

What This Calculator Does

Enter a starting balance, a nominal annual rate, and a compounding frequency (daily, monthly, quarterly, or annually). The calculator returns:

• Year-end balance for each of the next 1-10 years
• Total interest earned across the full term
• Effective annual yield (EAY) — the actual rate after intra-year compounding
• Owner-draw adjusted balance if you toggle that field
• After-tax value at your marginal rate

Quick Example: $25,000 at 5% Daily Compounding

• Year 1 end: $26,283.78
• Year 3 end: $29,037.99
• Year 5 end: $32,082.05
• Effective annual yield: 5.1267% (vs the nominal 5%)

The daily compounding adds $82 over 5 years compared to monthly compounding on the same balance. Not life-changing, but on a $250,000 reserve it is $820 over the same period.

Owner Draws and Reinvestment

A small business does not leave $25,000 untouched for 5 years. Toggle owner draw and set a monthly withdrawal (e.g. $1,000 for a quarterly owner distribution). The calculator subtracts draws before applying interest each month. It also flags when the projected balance would go negative — owner draws exceeding earnings — so you can plan a smaller draw or a top-up contribution.

Tax-Aware Projections

The after-tax field uses your marginal rate (federal + state, configurable in the params). The calculator distinguishes between:

• Tax-deferred accounts (SEP-IRA, Solo 401k): no tax applied
• Taxable business savings: interest is taxed as ordinary income at the marginal rate
• Tax-free accounts (Roth versions): no tax applied, but contributions are post-tax

The default is taxable business savings with a 24% marginal rate (override in the field). Most small-business reserves fall in this category.

Reinvestment Schedule

If you reinvest earnings (move them into a higher-yield instrument annually), the calculator accepts a reinvestment rate as a second field. The projection shows two curves: interest-only and interest + reinvested growth. This is the closest model to what a business reserve actually does in a real operating environment.

Common Pitfalls

• Using the nominal rate as the effective rate. The nominal 5% APR is not 5% APY when interest compounds intra-year. The calculator shows both side-by-side.
• Forgetting to subtract fees. Some business savings accounts charge a monthly fee. Add the fee as a negative contribution to model net growth.
• Mixing compounding schedules in the same projection. A 5% daily-compound account followed by a 4% monthly-compound account needs two separate calculations. Use the calculator’s “year-block” mode for this.

Limitations

The calculator does not model market risk. For business reserves invested in money-market funds with floating NAV, use the Inflation Calculator alongside to see real (inflation-adjusted) returns.

Related Tools

• Loan Calculator — interest owed, not earned
• Tax Calculator — quarterly estimated tax
• ROI Calculator — for one-off investments