A seller financing calculator is a model that tells you whether a deal with a seller note actually works — before you commit to it. Most buyers anchor on the total purchase price and the down payment. The number that actually determines whether you survive year one is annual debt service. A $200K seller note at 6% over 5 years adds $46,560 in annual payments. Stack that on top of your SBA payment and you need to know, with precision, whether the business's earnings cover it. Here's how to build and use a seller financing calculator on any deal.
The Key Numbers in Any Seller Financing Model
Before you can run the model, you need five inputs:
**1. Principal amount** — the total seller note. Typically 10–30% of purchase price. **2. Interest rate** — usually 5–8% for seller notes. Negotiate this if you're doing a large standby note. **3. Term** — usually 3–7 years. Shorter term = higher monthly payment but less total interest. **4. Standby period** — if combined with an SBA loan, the note may be on standby for 24 months (no payments during that period). **5. SDE or EBITDA** — what the business actually earns. This is what you're servicing the debt from.
With these five numbers, you can calculate the monthly payment, total interest cost, annual debt service, and whether the deal cash flows.
How to Calculate Monthly Seller Note Payments
The standard loan amortization formula gives you the monthly payment:
**P × [r(1+r)^n] / [(1+r)^n – 1]**
Where P = principal, r = monthly interest rate (annual rate ÷ 12), n = total number of monthly payments.
Let's apply it to a $250K seller note at 6% interest over 5 years: - P = $250,000 - r = 6% ÷ 12 = 0.5% = 0.005 - n = 60 months
Monthly payment = $250,000 × [0.005(1.005)^60] / [(1.005)^60 – 1] = $250,000 × [0.005 × 1.3489] / [1.3489 – 1] = $250,000 × 0.006745 / 0.3489 = $250,000 × 0.01933 = **$4,833/month** or **$57,996/year**
Total interest paid over the life of the note: $57,996 × 5 = $289,980 total payments – $250,000 principal = **$39,980 in interest.**
Knowing this number — not just the headline $250K — is what lets you model the deal accurately.
EBITDA Estimator
Estimate adjusted earnings to see how much debt service the business can actually support.
Estimate Earnings →Modeling the Full Debt Stack (SBA + Seller Note)
In a typical acquisition, you're not just modeling the seller note — you're modeling two debt obligations on top of each other.
Example: $1.5M acquisition, SBA at $1.2M (80%), seller note at $150K (10%), buyer equity $150K (10%).
**SBA payment:** $1.2M at Prime + 3% (currently ~11.5%), 10-year amortization. - Monthly: ~$16,588 / Annual: ~$199K
**Seller note:** $150K at 6%, 5 years, on 24-month standby. - Standby years 1–2: $0/year - Active years 3–5: ~$34,800/year
**Total debt service:** - Years 1–2: $199K (SBA only) - Years 3–5: $234K (SBA + seller note)
If the business generates $320K in SDE, your coverage ratio in year 1 is 1.61x. Excellent. In year 3, it drops to 1.37x. Still fine — the SBA threshold is 1.25x. But the margin is thinner. This is exactly why you model both obligations before you agree to terms.
Cash-on-Cash Return: What the Model Actually Shows You
Debt coverage tells you whether the deal survives. Cash-on-cash return tells you whether the deal is worth doing.
Cash-on-cash return = annual cash flow ÷ total equity invested
Using the example above: $320K SDE – $199K SBA debt service = $121K year-1 cash flow to the owner. Equity invested: $150K.
$121K ÷ $150K = **80.7% cash-on-cash return in year 1.** That's before any salary you pay yourself. If you pay yourself $80K (market rate for a similar management role), your owner cash flow is $121K and your return on the equity is still 80%.
By year 3, with the seller note active: $320K SDE – $234K total debt service = $86K. Your COC drops to 57%. Still strong — but you need to understand the trajectory before you're surprised by a smaller check in year 3.
This is the model every serious buyer runs before making an offer. The seller financing calculator isn't about the note in isolation — it's about the full cash picture across every year of the loan.
LOI Generator
Once the numbers work, build your offer with the seller note terms already structured.
Build Your LOI →What to Do When the Model Doesn't Work
If the model shows coverage below 1.25x or a cash-on-cash return under 20%, the deal either needs to be repriced or restructured — not forced.
**Lower the purchase price.** If the seller note and SBA payment together exceed what the business earns, the price is too high. Calculate the maximum purchase price that produces 1.25x coverage at your expected rate, and negotiate from there.
**Extend the seller note term.** A $250K note at 6% over 5 years costs $57,996/year. The same note over 7 years costs $44,436/year — saving $13,560 annually in debt service. That can be the difference between viable and marginal.
**Increase the standby period.** Push the seller note standby from 24 to 36 months if the SBA lender allows it. This improves cash flow in years 1–3 when transitions are hardest.
**Request a lower interest rate.** 4% vs 6% on a $300K note saves $3K per year. It's worth asking.
Deal Flow OS helps you find deals with the financial profiles that make this modeling exercise result in a yes — not a pass.
A seller financing calculator is just a debt amortization model applied to a deal — but most buyers skip it entirely and rely on gut feel. Don't. Know your payment, know your coverage ratio, know your cash-on-cash return before you write the LOI. That 20 minutes of modeling prevents years of regret.
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