How to Calculate Bridge Loan Cost When Buying Before Selling

A Bridge Loan Is a Short-Term, Interest-Only Debt Against Your Current Home's Equity

A bridge loan lets you borrow against your current home's equity to fund the down payment or full purchase of a new home before your existing home sells. The loan term is typically 6 to 12 months. Lenders charge interest only during that period. The principal is repaid in full when your original home closes.

The structure sounds clean. The cost calculation is not. You are simultaneously paying interest on the bridge loan, paying the mortgage on the home you have not yet sold, and paying the mortgage on the home you just purchased. All three obligations run concurrently until the old property closes.

The Core Formula for Bridge Loan Interest Cost

Total bridge loan interest equals the loan balance multiplied by the annual rate, divided by 365, multiplied by the number of days the loan is outstanding.

Written as plain text:

Bridge Interest = Loan Balance x (Annual Rate / 365) x Days Outstanding

This is the number most borrowers never compute before signing. They see the rate. They do not see the daily meter.

Worked Example 1: 90-Day Bridge Loan at 10.5%

A buyer needs a $400,000 bridge loan to cover a down payment on a $900,000 replacement home. The lender quotes 10.5% annual interest, interest-only. The old home is expected to sell in 90 days.

Daily interest: $400,000 x (0.105 / 365) = $115.07 per day

Total interest over 90 days: $115.07 x 90 = $10,356

Add the origination fee. Most bridge lenders charge 1% to 2% of the loan amount. At 1.5%, that is $6,000 on a $400,000 loan.

Add closing costs on the bridge itself: appraisal ($600), title search ($800), and recording fees ($200) are typical. Call it $1,600.

Total bridge loan cost for 90 days: $10,356 + $6,000 + $1,600 = $17,956

That is $17,956 before the buyer has paid a single dollar on either mortgage.

Dual-Carry Cost: The Expense Most Buyers Overlook

Dual-carry is the combined monthly mortgage obligation on both the outgoing and incoming properties. Bridge loan interest layers on top of this. The three-cost stack determines whether the transaction is actually affordable during the overlap window.

Worked Example 2: Full Three-Cost Stack on a 120-Day Overlap

Buyer details:

• Existing home mortgage: $3,100 per month (principal and interest, 30-year fixed at 6.25%)
• New home mortgage: $4,850 per month (30-year fixed at 7.1% on a $780,000 loan)
• Bridge loan: $350,000 at 10.75%, 120-day term

Bridge daily interest: $350,000 x (0.1075 / 365) = $103.08 per day

Bridge total interest over 120 days: $103.08 x 120 = $12,370

Monthly equivalent of bridge interest: $12,370 / 4 months = $3,092 per month

Monthly three-cost stack:

• Old mortgage: $3,100
• New mortgage: $4,850
• Bridge interest (monthly equivalent): $3,092

Total monthly cash obligation during overlap: $11,042

Origination at 1.5% on $350,000 adds $5,250 upfront. Total bridge loan cost including origination: $12,370 + $5,250 = $17,620.

A buyer with $18,000 in liquid reserves and a four-month timeline has essentially no margin for a delayed closing on the sale side.

How Loan Size and Duration Drive the Total Cost

Two variables dominate the final number: loan balance and days outstanding. Rate matters less than borrowers assume. A 0.5% rate difference on a $400,000 bridge loan held for 90 days produces only $493 in additional interest. A 30-day extension at the same rate costs $1,381 in additional interest on that same loan.

Extending the bridge term costs more than rate shopping.

If your sale timeline is uncertain, the conservative assumption is a 150-day term, not 90. Model both scenarios before you commit to the purchase.

What Lenders Do Not Volunteer in the Quote

Bridge lenders quote the annual rate. They do not automatically present the origination fee, the daily interest calculation, or the realistic overlap duration. Some structure the loan with a minimum interest clause, meaning you pay interest for a minimum of 60 or 90 days even if your home sells in 45 days. Read the term sheet for that clause before signing.

Also confirm whether the lender requires both properties to be on the title during the bridge period. Some lenders place a lien on the new property in addition to the old one, which affects your ability to refinance the new home after the sale closes.

When a Bridge Loan Makes Financial Sense

A bridge loan makes sense when the equity in your current home is substantial and the spread between purchase price and expected sale price is wide enough to absorb the full cost stack. Specifically, when the total bridge cost (interest plus fees) represents less than 1% of the expected equity released at sale, the math supports moving forward.

On a home selling for $850,000 with a $310,000 remaining mortgage, expected equity is $540,000 (before agent commissions and closing costs). A $17,956 bridge loan cost represents 3.3% of that equity. That is a meaningful but not prohibitive cost for a buyer who needs to act quickly in a competitive market.

The calculation changes if the home takes 180 days to sell instead of 90. Running both the expected and worst-case scenarios is not optional. It is the minimum analysis required before you sign a purchase agreement contingent on bridge financing.

Run Your Numbers Before You Sign Anything

The CalcMoney Mortgage Calculator lets you model the new home mortgage payment and compare it directly against your existing obligation. Use it to establish the fixed monthly costs before layering in the bridge interest calculation above. Enter your loan amount, rate, and term to get the monthly payment on the new property. Then apply the bridge formula manually using the figures on this page.

The sequence: calculate new mortgage payment, calculate old mortgage carry cost, calculate bridge daily interest times expected days, add origination and closing costs on the bridge. Sum all four. Compare that total against your liquid reserves and expected sale proceeds. If the margin is thin, the bridge loan is a risk, not a solution.

Open the CalcMoney Mortgage Calculator and run your scenario now →

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Results are estimates for informational purposes only. Consult a licensed financial professional before making financial decisions.