Cost of Debt Formula: What It Is, Why It Matters, and How to Calculate It

Most childcare businesses carry some form of debt — an SBA loan that funded the build-out, an equipment loan for a new playground, a line of credit that covers payroll during a slow summer, or a mortgage on the building. If any of that sounds familiar, you need to understand the cost of debt formula: what it is, what it tells you, and how to use it to make better decisions for your daycare.

Calculating your cost of debt can get complex quickly. So let’s start with the basics and build up to more nuanced examples. Once you understand the cost of debt formula, you can use it to your advantage — to negotiate better loan terms, plan growth, and make the case to investors or lenders that your childcare business is well run.

Getting the Information You Need for the Cost of Debt Formula

The term “cost of debt” sounds straightforward. In simple terms, it’s the amount you pay to carry debt — your interest payments on what you’ve borrowed to operate your childcare business.

In the real world, however, you need a little more information than that. In addition to your average interest rate, you need to know your marginal tax rate. Once you have those two numbers, you can apply the basic cost of debt formula:

Effective interest rate × (1 – marginal tax rate)

Keep in mind, this version of the cost of debt formula is still simplified. It assumes a single effective interest rate. In reality, your interest rates likely shift across the fiscal year, especially if you’re carrying multiple loans or a variable-rate line of credit. Don’t worry about that yet — you have to walk before you can run (Investopedia: Cost of Debt).

Calculating Your After-Tax Cost of Debt

If the formula left you scratching your head, real numbers help. Imagine your childcare center paid $20,000 in interest on a $500,000 SBA loan last year. Divide the annual interest by the total debt and multiply by 100:

$20,000 / $500,000 × 100 = 4%

Now you know your effective interest rate is 4%. To find your cost of debt, multiply by one minus your marginal tax rate. C-corporations currently face a 21% federal corporate tax rate (made permanent by the One Big Beautiful Bill Act of 2025), plus state taxes that vary widely. According to the Tax Foundation, combined federal and state rates for most corporations land in the 22–27% range.

For this example, assume a combined rate of 25%. Your after-tax cost of debt is:

4% × (1 – 0.25) = 3%

Plug your own numbers into the cost of debt formula and you’ll have a quick estimate. It won’t be perfect, but it gives you a solid working figure.

Getting More Specific With Your Cost of Debt

Estimates help in plenty of situations. However, when you need to know your true cost of debt, you have to dig deeper.

Your Loan Balance Changes Throughout the Year

Your loan amount probably falls throughout the year as you make payments. At the start, you paid interest on $500,000. By the last quarter, you may have been paying interest on $450,000 or $420,000, depending on your amortization schedule. As the balance falls, your cost of debt also drops slightly.

The good news: your real cost of debt usually looks smaller than the simple formula suggests. The bad news: you can’t do these calculations in your head. An amortization schedule (or amortization table) shows exactly how much you’ve paid in principal versus interest at any point. Free amortization calculators are available from Bankrate and most lender portals — there’s no need to build one from scratch.

You May Have Multiple Forms of Debt

One more wrinkle: your childcare business may carry multiple forms of debt. An SBA 7(a) loan funded the build-out. An equipment loan financed a passenger van two years later. A business line of credit covers payroll gaps during slow enrollment months. Each form of debt likely has a different principal and interest rate.

Before you apply the cost of debt formula, calculate your weighted average interest rate across all loans. Add up the interest you paid on each loan, divide by the total balance across all loans, and you have a single weighted average that reflects your actual borrowing cost.

Pay close attention to credit card debt. Most business credit cards charge well over 20% APR. At those rates, a $5,000 balance can cost you as much in interest as a $20,000 SBA loan at 5%. Keep credit card balances near zero, or your cost of debt formula result will look much worse than it needs to.

Figuring Out Your Tax Rate

You now have a clearer view of how much you spend on debt and a default average tax rate to work with. However, you probably don’t pay the average rate.

If you want your true cost of debt, you need to calculate your actual effective tax rate. Your tax rate also depends on how your childcare business is structured. C-corporations pay the 21% federal corporate rate. S-corporations, partnerships, and sole proprietorships are pass-through entities — meaning the business itself doesn’t pay federal income tax. Instead, the owners pay tax on the business’s income at their personal rates. The structure you chose has a major impact on your cost of debt formula result. For more on this, see our guide to choosing the right business structure for your childcare business.

The math itself is simple: total taxes paid divided by total taxable income gives you your effective tax rate. Filling out the forms to get there is where time and sweat come in. If your business is structured as a pass-through, work with your CPA to estimate your blended marginal rate across federal, state, and self-employment taxes.

Why Use the Cost of Debt Formula?

You now have the tools to calculate your cost of debt. But why bother? What’s the point of customizing an amortization table and pinning down your effective tax rate? Three big reasons.

You Save Money by Avoiding Excess Interest

If you’ve never calculated your cost of debt, you don’t know how much your business actually spends on interest. You might know your monthly loan payment, but that figure mixes principal and interest. Without separating them, you can’t see what your debt really costs.

Once you run the cost of debt formula, you’ll have a real number. You may discover the cost is high enough to be holding back your center’s growth. Few things sharpen priorities like seeing exactly how much money goes to interest each year. A clean financial dashboard makes this visibility automatic.

You Create Room for Growth and Expansion

Childcare businesses that don’t calculate their cost of debt miss critical information. They can’t build accurate budgets, plan capital expenses, or know when they’ll hit financial goals.

Can you afford to hire a new lead teacher? Add a classroom? Buy the building you’ve been leasing? You can’t answer those questions until you know your cost of debt. If you have growth plans, knowing the cost of your debt lets you set priorities that actually move the business forward. Tracking the right KPIs for your childcare business alongside your cost of debt gives you a complete picture.

You Make Your Business More Attractive to Lenders and Investors

Whether you’re applying for a bigger SBA loan, refinancing your building mortgage, or bringing on a partner, lenders and investors will ask about your cost of debt. They want to know that you understand your own numbers. A business owner who can’t speak to their cost of debt formula looks like a risk. One who can — confidently, with documentation — looks like someone worth backing.

Ways to Improve Your Cost of Debt

Since lowering your cost of debt opens up real opportunities, it’s worth working on. None of these levers will move overnight, but with patience and planning you can reduce what you spend on interest and free up capital for growth.

Lower Your Interest Rates

The lower your interest rates, the less you pay to your lenders. Even a couple of percentage points can save a meaningful amount of money. For example, on a $500,000 loan with a 5-year term, dropping your rate from 8% to 6% saves roughly $30,000 in interest over the life of the loan.

Borrowers often qualify for lower rates when they:

• Improve their business credit score by paying down balances and correcting credit-report errors
• Add a guarantor with strong personal credit
• Pledge collateral — typically the building, equipment, or accounts receivable
• Demonstrate consistent on-time payment history

If you have a strong relationship with your lender and a clean payment record, ask directly about a rate reduction. Even one late payment can damage that conversation, so consistency matters.

Repay Loans Faster

The less you owe, the lower your interest payments become. With current rates well above historical lows (track the latest at the St. Louis Fed), early repayment can save substantial money for many childcare businesses today. A $500,000 loan at 8% paid off in 3 years instead of 5 saves roughly $30,000 in interest.

Before you accelerate payments, check your loan documents for prepayment penalties. SBA loans typically have small or no prepayment penalties on shorter loans, but conventional commercial loans sometimes do.

Refinance Your Business Loan

A few years ago, your daycare may not have qualified for a low interest rate. If your business has grown and your financials have improved since then, refinancing can save real money. When you refinance, you borrow from a new lender to pay off the original debt — ideally at a lower rate or with better terms.

Refinancing also lets you consolidate. Instead of three loans with three lenders, one payment to one lender. You can also stretch the term to lower monthly payments — useful if cash flow is tight. Just remember: extending the term doesn’t lower your cost of debt unless your rate drops too.

Increase Revenue to Lower Your Effective Cost

Your cost of debt is fundamentally about proportionality — interest paid relative to revenue generated. You can lower the ratio by raising revenue. For a childcare business, that usually means filling open spots in your enrollment, raising tuition modestly, or adding a new classroom or service (extended care, summer programming, transportation).

This strategy only works if you use the new revenue to actually pay down debt. If you instead spend it elsewhere, you may grow your operations without reducing your cost of debt at all. For help planning growth investments against your debt picture, an outsourced CFO can run the scenarios alongside you. If you’re still working on your foundational plan, see our guide to the elements of an effective daycare business plan.

The Bottom Line on the Cost of Debt Formula

Nearly every childcare business carries some form of debt. Maintaining a reasonable cost of debt plays a major role in building a center that puts as much money as possible toward growth — not toward interest payments. When your debt costs too much, your business can’t grow. Instead, you exist to enrich your lender. The first step toward managing debt is knowing what it costs. Now that you understand the cost of debt formula, you can start moving your childcare business forward with informed decisions.

Want help running the cost of debt formula on your own books and building a plan to lower it? Schedule a discovery call with Honest Buck Accounting — our team can clean up your numbers, calculate your true after-tax cost of debt, and help you redirect cash toward growth. We can also act as your outsourced accountant if you need ongoing support.