Subject-To Financing Calculator: How to Run the Numbers on a Sub-2 Deal
Subject-to financing sounds deceptively simple: take over the seller's mortgage, get the deed, make the payments. You skip the bank, inherit a rate from 2021, and get a house with better terms than any lender will offer you today.
Not quite. A subject-to deal has inputs a conventional purchase doesn't, and most beginner calculators skip them. That's how investors end up owning a house where the existing loan balance is underwater relative to value — or where the monthly payment is $400 higher than they modeled because they forgot the escrow account was three months behind.
Here's how to actually run the numbers.
The Two Numbers That Aren't on the MLS
Every sub-2 deal starts with two numbers the listing won't show you:
1. The existing loan's actual payoff balance. Not the original loan amount. Not what Zillow estimates. The payoff statement from the servicer, which includes principal, accrued interest, and any escrow shortfall. Get this in writing before you commit to any deal terms.
2. The full PITI payment. Principal + Interest + Taxes + Insurance. The P&I comes from the note. But taxes and insurance live in an escrow account that may be behind — especially if the seller stopped paying or let the insurance lapse. A $1,400/month P&I can become $1,900/month PITI when you factor in a property tax bill that jumped 20% and insurance that needs to be re-bound from scratch.
If you don't have both numbers verified, you're not underwriting. You're guessing.
The Core Calculator Inputs
A complete subject-to calculator needs eight inputs. Most free tools give you four.
| Input | What it is | Where to get it |
|---|---|---|
| Purchase price | Cash you're paying above loan balance | Negotiation |
| Existing loan balance | Verified payoff, not estimate | Servicer payoff statement |
| Existing interest rate | Rate on the original note | Note itself or servicer |
| Remaining loan term | Months left on amortization | Servicer or amortization schedule |
| Current PITI payment | Full monthly debt service + escrow | Most recent mortgage statement |
| Escrow balance | How far behind (or ahead) escrow is | Servicer escrow analysis letter |
| Market rent | Comped against 3 nearby actuals | Active listings + recent leases |
| Vacancy + expense rate | Your honest operating assumption | Market data + your track record |
The last two are identical to any buy-and-hold deal. The first six are unique to sub-2 and the ones that bite people.
Running the Cash Flow Model
Here's a real deal structure (numbers rounded, structure exact):
Property: 3/2 in a mid-sized Sunbelt market
ARV: $285,000
Existing loan balance (verified payoff): $198,000
Existing rate: 3.25%, 30-year fixed, originated 2021
Remaining term: 294 months
Current PITI: $1,440/month
Cash to seller: $8,500 above their payoff
Investor's out-of-pocket at close:
- Seller payment: $8,500
- Closing costs (title, attorney): ~$3,200
- Deferred maintenance: $4,800
- Total all-in: ~$16,500
Monthly model:
- Market rent: $1,950/month
- Vacancy (8%): –$156
- Effective gross income: $1,794
- Operating expenses (taxes, insurance, mgmt, repairs, capex reserve): –$780
- NOI: $1,014
- Existing PITI: –$1,440
- Monthly cash flow: –$426
That deal is negative. It fails cash-flow underwriting even with a 3.25% rate.
Why? Because the PITI swallowed the NOI. The existing debt service was set when rents in this market were $1,600. They've moved up, but not enough to rescue a payment that included PMI and a topped-up escrow cushion. A low rate does not equal good cash flow. The payment has to fit the rent roll.
If you want a baseline for reading NOI and expense ratios before adding the sub-2 layer, the Real Estate Underwriting 101 guide covers the foundation.
When the Math Works — and When It Doesn't
Sub-2 deals cash-flow well under specific conditions:
When it works:
- Seller's PITI is ≤ 40% of market rent. A $1,200 payment on a $3,000/month rent unit leaves real margin.
- The existing loan is small relative to value. Seller has substantial equity and a genuine motivation to exit (relocation, divorce, estate situation).
- You're in a rent-growth market. Even if Year 1 cash flow is thin, the payment is fixed and rent compounds.
- You're holding long. Sub-2 returns improve sharply at Year 5–7 when rents have grown 15–20% but the payment hasn't moved.
When it doesn't work:
- The existing loan is FHA or VA. Both have active enforcement mechanisms and servicers that flag due-on-sale triggers. The risk profile changes completely.
- PITI > 50% of gross rent. One bad month wipes out a quarter of cash flow.
- The seller isn't genuinely motivated. They need equity now, not a drawn-out creative structure.
- Your exit is a quick flip. You lose the rate advantage the moment you resell, and you've added legal complexity for zero holding-period gain.
If you're unsure whether a deal makes the cut, post it on /share-a-deal — other investors will spot the structural gaps before you commit capital.
The Equity Position: What You Actually Own
Most beginner guides skip this calculation. Your equity on day one is not ARV minus loan balance:
Equity = ARV − Existing Loan Balance − Your Cash In − Due-On-Sale Risk Buffer
That last term is real. If the servicer calls the note due — which they legally can under the due-on-sale clause, even if they rarely do — you need to refinance immediately at whatever rates exist at that moment. In today's market, that's a 7%+ conventional loan. Your equity cushion needs to survive that scenario.
For the example above:
- ARV: $285,000
- Existing loan balance: $198,000
- Cash in: $16,500
- Due-on-sale risk buffer (10% of ARV): $28,500
- Net equity position: ~$42,000
That's a real equity cushion worth protecting. If the number comes out thin — say, under $25,000 on a $250K ARV property — the deal needs to be renegotiated or passed. You don't have room to absorb a forced refi.
The Exit Math
Sub-2 investors have three exits. Know which one you're targeting before you close:
1. Hold and cash flow. Works when the payment-to-rent ratio is right and you're in a stable or growing market. The base case. Every year that passes, your payment stays fixed while rent drifts up.
2. Sell the property subject-to. Pass the existing loan and deed to the next buyer, capturing your equity spread. Requires a motivated buyer who understands the structure and a title company that will close it. Legal in most states; verify with your attorney for your jurisdiction.
3. Refinance out. You end the sub-2 structure and replace it with a new loan. Do this when rates drop meaningfully below the existing rate, when you need to pull equity, or when due-on-sale pressure forces your hand. The refinance payoff must pencil against your cash-in basis.
Year 5 hold scenario on the example property:
- Loan balance at Month 60 (5 years of amortization at 3.25% on 294 remaining months): ~$183,800
- Projected ARV at 3% annual appreciation: ~$330,000
- Gross equity at sale: $330K – $183.8K – 3% closing costs ≈ $136,000
That's approximately an 8× equity multiple on a $16,500 cash-in over 5 years. But only if the cash flow is sustainable, or you have the stomach to hold through a negative year while rents catch up to the payment.
The One Ratio That Filters in 30 Seconds
Before you build any model, run the payment-to-rent ratio:
PTR = Existing PITI ÷ Market Rent
- PTR < 35%: Strong. Worth building out the full calculator.
- PTR 35–50%: Marginal. Cash-flow depends on expense discipline and realistic vacancy.
- PTR > 50%: Skip it. The rate advantage doesn't compensate for the margin compression.
This is not a substitute for full underwriting — it's a 30-second filter that keeps you from spending two hours on deals that can't work arithmetically.
For deals that clear the PTR filter, dre1mery.com handles sub-2 deal structure natively: enter the existing loan terms, and the analyzer generates the full amortization schedule with cash-flow projections updating in real time as you adjust rent and expense assumptions.
If you're evaluating deals in a market you don't know well, finding an experienced local sub-2 investor to sanity-check your rent comps is worth the conversation. And before you layer on the creative structure, make sure the base rental math holds — how to underwrite a rental property covers the five metrics every buy-and-hold deal has to clear.
Subject-to financing is one of the most effective tools in a creative investor's toolkit. The existing loan rate is an asset. The existing loan payment is a constraint. Model both, know your equity cushion, and you'll make cleaner decisions on every sub-2 deal that crosses your desk.