How Business Line of Credit Interest Works

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Most business owners I worked with as a commercial loan officer assumed a line of credit charged interest the way a term loan does — borrow the whole amount, pay interest on the whole amount. That is not how it works, and the difference can save you real money.

A business line of credit is revolving credit. You get approved for a limit, you draw what you need, and you pay interest only on the dollars you’ve actually drawn — not on the full credit limit sitting available. Understand that one mechanic and the rest of the math follows.

This guide walks through how interest is actually calculated on a revolving line: how APR works, the difference between simple and compounding interest, why your rate moves with the prime rate, how draw fees differ from interest, and one fully worked (illustrative) example so you can see the moving parts. Every dollar figure below is an example only — never a quoted rate.

You only pay interest on what you draw

This is the single most important thing to understand, so I’ll say it plainly.

If you’re approved for a $50,000 line and you draw $10,000, you pay interest on $10,000 — not $50,000. The other $40,000 sits available, costing you nothing in interest until you draw it. (Some lenders charge a small annual or monthly maintenance fee on an open line regardless of use — that’s a fee, not interest. More on that below.)

This is what makes a line of credit different from a term loan. With a term loan, the full amount lands in your account on day one and interest starts accruing on all of it immediately. A line of credit only charges you for what you use, when you use it. That’s why lines are the standard tool for uneven, short-term needs — covering payroll between invoices, buying inventory ahead of a busy season, bridging a slow month.

Key terms, defined:

• Revolving credit — credit you can draw, repay, and draw again, up to your limit, without reapplying. As you pay down the balance, that room becomes available to borrow again.
• Draw period — the window during which you can take draws from the line. Some lines stay open indefinitely; others have a defined draw period followed by a repayment period.
• Draw — a single withdrawal from your available credit.

How APR is calculated on a revolving line

APR (annual percentage rate) is the yearly cost of borrowing, expressed as a percentage. On a revolving line, the lender doesn’t charge you a year’s worth of interest at once — they break that annual rate down to the period you actually carry a balance.

Here’s the mechanic lenders use under the hood:

1. Take the APR and convert it to a daily or monthly periodic rate. For a daily rate, that’s the APR divided by 365. For a monthly rate, it’s the APR divided by 12.
2. Apply that periodic rate to your outstanding balance — the amount you’ve drawn and not yet repaid.
3. Charge that interest for each day (or each billing period) the balance is outstanding.

So your interest cost depends on three things: how much you drew, the rate, and how long you carried the balance. Pay a draw back quickly and you pay very little interest, even on a high APR. Carry it for months and the cost climbs.

One caution: APR on a line of credit and APR on a fixed term loan aren’t always built the same way, and short-term lenders sometimes quote pricing in formats that look cheaper than they are. We cover that comparison in factor rate vs. APR and in average rates and fees.

Simple interest vs. compounding (periodic) interest

There are two ways the interest math can play out, and which one applies changes your total cost.

Simple interest is charged only on the principal you drew. If interest doesn’t get added back into the balance, you’re paying the same base each period until you pay it down.

Compounding interest is charged on the principal plus any unpaid interest that’s been added to your balance. When interest “compounds,” unpaid interest from one period becomes part of the balance the next period’s interest is calculated on. The more often it compounds — daily compounds faster than monthly — the more it adds up if you carry a balance.

Many business lines accrue interest daily on the outstanding balance and bill monthly. Whether that effectively compounds depends on the lender’s terms and whether you’re paying the interest each cycle, and the accrual and compounding convention varies by lender.

The practical takeaway doesn’t depend on the fine print: the faster you repay a draw, the less interest accrues, under either method. If you’re comparing two lenders, ask each one directly: is interest simple or compounding, and how often does it compound? Get the answer in writing.

Variable rates and the prime rate

Most business lines of credit carry a variable rate, which means your rate can move up or down over time. It’s not random — it’s tied to a published benchmark, most commonly the prime rate.

Prime rate is a baseline interest rate that commercial banks use as a reference for lending to their most creditworthy customers. It moves with the federal funds rate set by the Federal Reserve. When the Fed raises or lowers rates, prime typically follows, and variable-rate lines move with it.

Lenders usually quote a variable rate as prime plus a margin — for example, “Prime + a margin,” where the margin reflects your business’s risk profile (time in business, revenue, credit, collateral). Stronger borrowers get a smaller margin. The published prime rate is available from the Federal Reserve and is widely reported — it stood at roughly 6.75%* in mid-2026, though it moves whenever the Fed changes rates, so check the current figure before you budget. The margin is set by the lender and is where your business’s qualifications actually show up.

* Illustrative — the prime rate changes whenever the Federal Reserve adjusts rates. Confirm the current published figure before you budget.

What this means for you: your monthly interest cost can change even if you never draw another dollar, because the underlying prime rate can change. Budget with a little cushion if you’re carrying a balance on a variable line. If you want rate certainty, ask whether a fixed-rate option exists — availability varies by lender, and many lines don’t offer one.

Draw fees vs. interest — they’re not the same

Interest is the cost of carrying a balance over time. Fees are separate charges, often flat, that don’t depend on how long you borrow. Confusing the two leads people to underestimate the real cost of a line.

Common fees you may encounter on a line of credit:

• Draw fee — a flat fee or small percentage charged each time you take a draw. On a line you tap frequently, draw fees can add up faster than the interest does.
• Maintenance / monthly fee — a recurring fee to keep the line open, charged whether or not you’ve drawn anything.
• Origination / opening fee — a one-time fee to set up the line.
• Annual fee — a yearly charge to keep the line available.

Not every lender charges every fee — some charge none of these, some charge several. Because fees aren’t part of the interest rate, two lines with the same APR can cost very different amounts once fees are in. When you compare offers, look at APR plus the full fee schedule together, not the headline rate alone. Our average rates and fees page breaks down which fees to watch for.

A worked example (illustrative only)

Here’s how the pieces fit together. Every number below is a made-up example to show the math — it is not a quoted rate, not an offer, and not representative of what you’d be approved for. Confirm real terms with a lender.

The 18% APR, 2% draw fee, and 365-day daily-rate basis above are illustrative placeholders chosen to make the math clear — not a quoted rate or offer. Some lenders use a 360-day basis instead; confirm your lender’s actual rate, fee schedule, and day-count convention before you borrow.

What the example shows:

• Interest scales with balance × rate × time — change any one and the cost moves.
• The draw fee is fixed regardless of how long you borrow, so on short draws it can dwarf the interest.
• Paying back faster is the most reliable lever you control.

Want to run your own numbers? Use our business line of credit calculator.

Interest vs. factor rate — know which one you’re being quoted

Not every short-term financing product uses interest at all. Some — notably merchant cash advances and certain short-term products — price with a factor rate instead.

A factor rate is a flat multiplier (for example, an advance of a certain amount times a factor like 1.3) that determines your total payback up front. Critically, a factor rate does not decrease when you pay early the way interest does, and it isn’t an APR — so a factor rate and an APR can’t be compared side by side without converting them. A product quoted “1.3” can carry an effective APR far higher than it sounds.

How to minimize interest on your line of credit

The interest math gives you a clear playbook. None of this is exotic — it’s just discipline.

1. Draw only what you need, when you need it. Idle availability costs nothing in interest. There’s no benefit to drawing “extra to be safe” and parking it.
2. Repay draws as fast as your cash flow allows. Because interest accrues on the outstanding balance over time, every day you shave off a balance is interest you don’t pay. This is the biggest lever.
3. Watch draw fees on frequent small draws. If you tap the line constantly, a flat per-draw fee can cost more than the interest. Sometimes one larger draw is cheaper than several small ones — do the math.
4. Match the tool to the need. A line of credit shines for short-term, revolving needs. For a large one-time purchase you’ll repay over years, a term loan may carry less total interest.
5. Mind the variable rate. If you’re carrying a balance and the prime rate is rising, the cost of that balance rises too. Prioritize paying down variable-rate balances when rates are climbing.
6. Read the fee schedule before you sign, not after. Two lines with the same APR can cost very differently once maintenance, draw, and origination fees are in. Get it all in writing.

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By Marcus Delaney, former commercial loan officer. Reviewed by Elaine Vasquez for accuracy and compliance. BizBee is informational and independent. We are not a lender and do not broker loans. Some links are affiliate links — see How We Make Money.