1031 Exchange Calculator: Including the Debt Trap Most Miss

Introduction

Most 1031 calculators ask what you sold for and what you bought for, and tell you the exchange is fully deferred. That is only half the test. You can buy a more expensive property, receive no cash whatsoever, and still owe tax.

TL;DR: There are two kinds of boot. Cash boot is money left over. Mortgage boot is debt relief you did not replace — and it is taxable even though nothing reached your pocket. On the defaults, paying off a $200,000 mortgage and taking only $100,000 of new debt creates $100,000 of mortgage boot and $32,800 of tax, on an exchange that otherwise defers $100,480.

The formula

Start with the gain:

Net sale price = sale price
               − costs of sale

Adjusted basis = original basis
               − depreciation claimed

Total gain     = net sale price
               − adjusted basis

The gain splits into two pieces taxed at different rates:

Recapture gain = depreciation claimed,
                 capped at total gain
                 → 25% federally

Capital gain   = total gain
                 − recapture gain
                 → 0/15/20% federally

Then the two boot tests, and you must pass both:

equity out    = net sale price
                − mortgage paid off
equity in     = replacement price
                − new debt

Cash boot     = equity out − equity in
Mortgage boot = mortgage paid off
                − new debt

Taxable boot  = greater of the two,
                capped at total gain

Worked example

The defaults: selling for $650,000 with $45,000 of costs. Original basis $350,000, $80,000 of depreciation claimed, $200,000 mortgage paid off. Buying at $700,000 with $250,000 of new debt. Rates: 20% capital gains, 25% recapture, 3.8% NIIT, 5% state.

The gain:

• Net sale price: $650,000 − $45,000 = $605,000
• Adjusted basis: $350,000 − $80,000 = $270,000
• Total gain: $335,000
• Of which recapture: $80,000 (taxed at 25% + 3.8% + 5% = 33.8%)
• Of which capital gain: $255,000 (taxed at 20% + 3.8% + 5% = 28.8%)
• Tax if you simply sold: $100,480

The boot tests:

• Equity out: $605,000 − $200,000 = $405,000
• Equity in: $700,000 − $250,000 = $450,000
• Cash boot: $405,000 − $450,000 = negative, so $0
• Mortgage boot: $200,000 − $250,000 = negative, so $0

Both tests pass. $100,480 of tax deferred, nothing due.

The debt trap

Now change one number. Same $700,000 replacement property, but you take only $100,000 of new debt instead of $250,000 — perhaps because rates rose, or the lender's DSCR test came back tighter than expected.

• Mortgage boot: $200,000 − $100,000 = $100,000
• Tax on that boot: $32,800

You bought a more expensive property. You received no cash. You owe $32,800.

The logic is that the IRS treats debt relief as a benefit received. Walking away from a $200,000 obligation and only taking on $100,000 means you are $100,000 better off, and that is boot whether or not any cash moved.

Boot is taxed least-favourably-first — recapture before capital gain — which is why the $32,800 works out at 32.8% rather than the 28.8% capital gains blend.

The practical rule: replace both the value and the debt. If you cannot get the debt, you can make up the difference with additional cash into the purchase, which raises your equity in and neutralises the boot. What you cannot do is discover this at closing.

The two deadlines

Neither is negotiable and neither can be extended for convenience:

45 days from the closing of your sale to formally identify replacement properties, in writing, to your qualified intermediary. You may identify up to three regardless of value, or more under the 200% rule.

180 days from that same closing to complete the purchase — or the due date of your tax return for that year, whichever is earlier. That second clause catches late-year sales: sell in November and your 180 days can be truncated by the April filing date unless you extend.

Both clocks start on the sale closing, not on the identification. So a slow 45-day period does not buy you extra time at the back end.

The rules that disqualify an exchange

You cannot touch the money. Proceeds must go to a qualified intermediary, not to you. Constructive receipt — including having it briefly land in your account — disqualifies the exchange entirely. This is the most common fatal error.

Both properties must be held for investment or business use. A primary residence does not qualify. Neither does property held primarily for resale, which is why flips are not exchangeable.

Real property only. Since 2018, personal property no longer qualifies. Machinery, vehicles and — relevantly for a furnished short-term rental — the furniture, are excluded, which can create a small amount of unavoidable boot on an STR exchange.

Related-party rules apply. Exchanging with a related party triggers a two-year holding requirement on both sides, with limited exceptions.

Where this calculator is deliberately simple

One relinquished property, one replacement. Multi-property and reverse exchanges are common and are beyond a single page.

No partial-exchange proration of basis. Where boot exists, the replacement property's basis calculation gets more involved than shown here.

Federal rate bands are inputs, not looked up. Your capital gains rate depends on total taxable income, and NIIT only applies above an income threshold. Enter your actual rates.

State treatment varies more than you would like. Some states have clawback provisions that tax deferred gain when you eventually sell out of state. California in particular tracks deferred gain indefinitely.

This is not tax advice. A 1031 requires a qualified intermediary engaged before closing, and the rules are unforgiving of good-faith mistakes.

FAQ

What is boot in a 1031 exchange?

Anything of value you receive that is not like-kind replacement property. Cash boot is leftover proceeds; mortgage boot is debt relief you did not replace. Only the greater of the two is taxed, and only up to your total gain — but either one alone is enough to create a tax bill.

Can I owe tax on a 1031 exchange even if I received no cash?

Yes, and this is the trap. If you pay off more debt than you take on, the difference is mortgage boot and it is taxable. Replacing both the value and the debt of the property you sold is what makes an exchange fully deferred — buying up in price alone is not sufficient.

What are the 45 and 180-day rules?

45 days from the sale closing to identify replacement properties in writing to your intermediary, and 180 days from that same closing to complete the purchase — or your return's due date, whichever comes first. Both clocks start at the sale, and neither can be extended.

Does a 1031 exchange defer depreciation recapture?

Yes. Both the capital gain and the unrecaptured Section 1250 gain defer, which matters if you have claimed accelerated depreciation through a cost segregation study — the recapture that would otherwise be due at sale rolls forward instead.

Can I do a 1031 exchange on a property I flipped?

No. Property held primarily for resale is excluded — it is inventory rather than an investment asset. The holding period is not the only test, but a quick flip will not qualify regardless of how it is characterised.

What happens to the deferred tax eventually?

Either you pay it when you finally sell without exchanging, or you continue exchanging indefinitely. Under current law, property passing to heirs receives a stepped-up basis, which is why "swap till you drop" is a genuine estate strategy rather than a joke. That treatment is a policy choice and could change.

Do I need a qualified intermediary?

Yes, and you must engage them before the sale closes. If proceeds reach you or your agent even momentarily, the exchange fails. This is not a formality that can be corrected afterwards.

Conclusion

Run both boot tests before you go under contract on the replacement, not after. If your new debt is coming in lighter than the loan you are retiring, you have two options — more cash into the purchase, or a tax bill — and only one of them is a choice you get to make in advance.