How to Calculate HYSA Interest and What You Are Actually Earning

The APY Number on the Bank's Website Is Not Your Return

Banks advertise APY. That figure already accounts for daily compounding. But it assumes three things that are almost never true for any real account holder: a constant balance for 365 days, no rate changes during the year, and interest credited daily without exception.

Real accounts receive deposits and withdrawals throughout the year. Rates on HYSAs are variable. Most banks credit interest monthly, not daily, even when they compound daily. The gap between what the advertisement implies and what you receive is not a rounding error. On a $250,000 cash position, even a 0.10% difference in effective yield equals $250 per year.

Understanding the actual formula closes that gap.

The Core Formula: Daily Compounding

Every major HYSA uses daily compounding. The bank calculates interest on your balance every single day using the daily periodic rate.

Daily Periodic Rate (DPR) = APR / 365

At an APR of 4.75%, the DPR is 0.013014%.

The ending balance formula after n days:

Ending Balance = Principal × (1 + DPR)^n

Or expressed with the full APR:

Ending Balance = Principal × (1 + APR/365)^365

That produces the true APY:

APY = (1 + APR/365)^365 - 1

At 4.75% APR, the APY is 4.8635%. On a $100,000 deposit held for exactly one year, the difference between using APR and APY is $96.50. Banks advertise APY precisely because it is the larger number.

Worked Example 1: $75,000 Held for 12 Months

Assume a starting balance of $75,000, a stated APY of 4.85%, and no deposits or withdrawals for exactly 365 days.

Step 1: Identify the APR.

Work backward from APY:

APR = 365 × ((1 + APY)^(1/365) - 1)

APR = 365 × ((1.0485)^(1/365) - 1) = 365 × 0.0001298 = 4.7377%

Step 2: Apply the daily compounding formula.

Ending Balance = $75,000 × (1 + 0.047377/365)^365

Ending Balance = $75,000 × 1.0485

Ending Balance = $78,637.50

Total interest earned: $3,637.50

Step 3: Verify against the simpler APY shortcut.

$75,000 × 0.0485 = $3,637.50. The shortcut works only when the balance is constant for exactly one year. The moment you add or withdraw funds, it breaks down.

Worked Example 2: Variable Balance Across 6 Months

This is closer to how accounts actually work. You start with $40,000, add $15,000 on day 45, and withdraw $10,000 on day 120. The APY is 4.75%. You want to know what you earn through day 180.

Break the calculation into three segments.

Segment 1: Days 1 through 44 (44 days, $40,000 balance)

DPR = 4.8635% / 365 = wait. Use APR for the daily calculation.

APR implied by 4.75% APY = 365 × ((1.0475)^(1/365) - 1) = 4.6393%

DPR = 4.6393% / 365 = 0.012710% per day

Balance after day 44:

$40,000 × (1 + 0.00012710)^44 = $40,000 × 1.005596 = $40,223.84

Segment 2: Days 45 through 119 (75 days, $55,223.84 balance after deposit)

Balance after deposit on day 45: $40,223.84 + $15,000 = $55,223.84

Balance after day 119:

$55,223.84 × (1 + 0.00012710)^75 = $55,223.84 × 1.009563 = $55,751.77

Segment 3: Days 120 through 180 (61 days, $45,751.77 balance after withdrawal)

Balance after withdrawal: $55,751.77 - $10,000 = $45,751.77

Balance after day 180:

$45,751.77 × (1 + 0.00012710)^61 = $45,751.77 × 1.007790 = $46,108.22

Total interest earned over 180 days: $46,108.22 - $40,000 - $15,000 + $10,000 = $1,108.22

Running this same scenario with a naive half-year APY estimate ($65,000 average balance × 4.75% / 2 = $1,543.75) overstates your return by $435.53. The average balance shortcut is imprecise when the timing of deposits and withdrawals is uneven.

Why the Rate Change Problem Matters More Than You Think

HYSAs are variable rate products. The Federal Reserve has moved rates multiple times in single calendar years. In 2023, the fed funds rate changed four times. In 2024, it changed three times. Each time the Fed moves, your HYSA rate follows within days.

That means an account advertising 4.75% APY in January might pay 4.25% by October. Your blended yield for the year is not 4.75%. It is a time-weighted average of the rates in effect on each day your balance sat in the account.

The correct approach: treat each rate period as a separate compounding segment, exactly like the variable balance example above. Substitute rate change dates for deposit or withdrawal dates. Run each segment independently, then chain the ending balances forward.

On a $200,000 balance with a rate cut of 0.50% on day 180, the difference between assuming the full-year rate held versus calculating the true blended rate is approximately $500 in overstated earnings projections. That is not a planning error you want embedded in a cash flow model.

The Tax Layer: What You Actually Keep

Interest from HYSAs is ordinary income. It is not taxed at the preferential 15% or 20% rate that applies to qualified dividends and long-term capital gains.

At a 37% federal marginal rate plus a 5% state income tax rate, a 4.75% APY on $100,000 produces:

• Gross interest: $4,750
• Federal tax at 37%: $1,757.50
• State tax at 5%: $237.50
• Net after-tax return: $2,755
• Effective after-tax yield: 2.755%

Compare that to a tax-exempt municipal money market fund yielding 3.10%. The muni fund delivers $3,100 in after-tax income on the same $100,000 for investors in the 37% bracket. The HYSA looks better on the bank's website. The muni fund puts more money in your account.

Always calculate the tax-equivalent yield before deciding between taxable and tax-exempt cash instruments.

Tax-Equivalent Yield = Tax-Exempt Yield / (1 - Marginal Tax Rate)

A 3.10% muni yield for a 37% bracket investor has a tax-equivalent yield of 3.10% / 0.63 = 4.921%. That beats the HYSA's 4.75% APY.

How Banks Credit Interest: Monthly vs. Daily

Most HYSAs compound interest daily but credit it monthly. This distinction affects when your balance grows and therefore when compounding begins working on the newly added interest.

Daily compounding with monthly crediting means your balance on the statement date jumps by roughly one month's worth of interest. That credited amount then participates in the next month's compounding. The math is not materially different from true daily compounding at these rate levels, but the timing affects intra-month balance tracking.

If you withdraw funds before the monthly credit date, you may lose that month's accrued interest. Check your account agreement. Some banks pay accrued interest on withdrawal. Others do not.

What Your Bank's Interest Statement Is Actually Telling You

Your monthly statement shows interest earned. It does not show your effective yield. To calculate your actual yield for any statement period:

Effective Monthly Yield = Interest Credited / Average Daily Balance

Annualized Yield = ((1 + Monthly Yield)^12 - 1) × 100

If your statement shows $312.50 earned on an average daily balance of $80,000:

Monthly Yield = $312.50 / $80,000 = 0.3906%

Annualized Yield = ((1.003906)^12 - 1) × 100 = 4.787%

That is your actual earned yield, not the advertised rate. If the bank advertised 5.00% APY and you earned 4.787%, the difference reflects either mid-month balance changes, a rate reduction, or interest methodology you did not account for.

Run the Exact Numbers on Your Account

The formulas above produce precise results when you apply them to your actual balance history. Most people do not do this. They accept the advertised APY as their return, ignore rate changes, and never calculate the after-tax yield.

That approach may be costing you hundreds of dollars per year in mispriced decisions. Choosing the wrong account, withdrawing at the wrong time, or failing to compare tax-equivalent yields on alternatives all compound quietly.

The CalcMoney Savings Calculator handles all of this. Enter your balance, your rate, your deposit schedule, and your tax bracket. The calculator returns your projected gross earnings, your after-tax return, and a month-by-month interest schedule.

Calculate your actual HYSA return now →

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The advertised rate is a starting point. Your real return requires the math.