A seller finance calculator is the model you use to answer one question before making an offer: does this deal actually cash flow? Most buyers can intuit that seller financing reduces their cash at close. Fewer run the full numbers — principal, interest, term, combined with the SBA payment, divided into the business's earnings — to see whether the structure actually works before they commit. This guide walks through the math so you can do it on any deal in 15 minutes.
The Five Variables Every Seller Finance Calculation Needs
You can't run a meaningful seller finance model with fewer than five inputs:
**1. Seller note principal.** The amount the seller is carrying. On a $1.5M deal, a 15% seller note is $225K.
**2. Interest rate.** Typically 5–8% for seller notes in small business deals. 6% is a common midpoint.
**3. Term.** Usually 3–7 years. Standard is 5 years.
**4. Standby period.** If combined with SBA, the note is often on 24-month standby — no payments for two years.
**5. Business SDE/EBITDA.** What the business actually earns. This is what services all debt.
With these five numbers, you calculate: monthly payment, annual payment, year-by-year cash flow, and debt service coverage ratio. That's the full picture.
How to Calculate the Seller Note Payment
The monthly payment formula is standard loan amortization:
**Monthly Payment = P × [r(1+r)^n] / [(1+r)^n – 1]**
Where: - P = principal ($225,000) - r = monthly interest rate (6% annual ÷ 12 = 0.5% = 0.005) - n = number of months (5 years = 60 months)
Plugging in: = $225,000 × [0.005 × (1.005)^60] / [(1.005)^60 – 1] = $225,000 × [0.005 × 1.3489] / [0.3489] = $225,000 × 0.01933 = **$4,349/month** or **$52,191/year**
Total interest paid over 5 years: $52,191 × 5 – $225,000 = **$35,955**
Knowing the annual cost of the seller note — $52K/year in this example — is what lets you test whether the deal cash flows when stacked against your SBA payment.
EBITDA Estimator
Estimate the business's adjusted earnings before stacking them against your debt model.
Estimate Earnings →The Full Deal Model: SBA + Seller Note + Business Earnings
Here's a complete model for a $1.5M acquisition:
**Deal structure:** - Purchase price: $1,500,000 - SBA loan (80%): $1,200,000 at 12%, 10 years → $171,720/year - Seller note (10%): $150,000 at 6%, 5 years, 24-month standby → $34,794/year (years 3–5) - Buyer equity (10%): $150,000 cash
**Business earnings:** $340,000 SDE (trailing 12 months)
**Year 1–2 (SBA only, seller note on standby):** Debt service: $171,720 Coverage ratio: $340K ÷ $171.7K = **1.98x** — excellent Owner cash available after debt: $168,280
**Year 3–5 (SBA + seller note active):** Debt service: $171,720 + $34,794 = $206,514 Coverage ratio: $340K ÷ $206.5K = **1.65x** — solid Owner cash available after debt: $133,486
Both periods comfortably exceed the 1.25x SBA threshold. The seller note standby makes years 1–2 particularly strong. This is a deal that works.
Cash-on-Cash Return: Is the Deal Worth Your Equity?
Debt coverage tells you whether the bank approves. Cash-on-cash return tells you whether the deal makes financial sense for you.
**Cash-on-cash return = year 1 free cash flow ÷ equity invested**
Using the example above: - Year 1 free cash flow (SDE minus SBA payment): $340K – $172K = $168K - Equity invested: $150K - Cash-on-cash return: $168K ÷ $150K = **112%**
That's a 112% return on your equity in year one — before any salary you pay yourself. Even if you take $80K as salary (counted within the SDE), your unlevered return on the equity is 112% annually.
Now compare to a deal where the seller note is NOT on standby — both payments start year 1: - Year 1 debt service: $171.7K + $34.8K = $206.5K - Free cash flow: $340K – $206.5K = $133.5K - Cash-on-cash: $133.5K ÷ $150K = **89%**
Still excellent. But the 24-month standby on the seller note is worth 23 percentage points of return in year one. That's why you fight for the standby period when negotiating.
When the Seller Finance Model Doesn't Work
Not every deal passes the coverage test. Here's how to identify the break-even points and what to do when a deal fails the model.
**Maximum purchase price given earnings.** Work backward from the coverage test: if SDE = $280K and minimum coverage = 1.25x, maximum annual debt service = $280K ÷ 1.25 = $224K. If SBA payment alone exceeds $224K, the deal is too expensive. Either the price needs to come down, or the seller note needs to go on standby to reduce the active payment.
**Minimum SDE required for your deal structure.** If you're modeling a $1.2M acquisition with SBA at 12%, minimum SDE to hit 1.25x coverage is approximately $214K ($171K ÷ 0.8). If trailing SDE is below that, the deal doesn't qualify for the financing you're planning.
**Fixes when the model fails:** - Negotiate a lower price (the most direct fix) - Push for a longer seller note term (7 years instead of 5 reduces annual payment) - Push for a longer standby period (3 years instead of 2) - Negotiate a lower interest rate on the seller note - Bring more equity and reduce the SBA principal
Deal Flow OS helps you find deals where the numbers work before you spend time building the model — businesses with consistent earnings, manageable asking prices, and owners who are motivated to structure a deal.
LOI Generator
When the model works, build your offer with seller note terms already in the LOI.
Draft Your LOI →A seller finance calculator is 20 minutes of math that tells you whether a deal is worth pursuing before you spend months on due diligence. Run the full model — SDE, SBA payment, seller note payment, coverage ratio, cash-on-cash return — on every deal before you submit a LOI. The math doesn't lie.
Find Deals That Pass the Numbers Test
Deal Flow OS surfaces businesses with the financial profiles that make seller finance models work.
Start Your Free 7-Day Pro Trial