A construction loan does not behave like a normal mortgage during the build. You usually pay interest only on what has actually been drawn, which means the monthly number starts smaller and grows as the project moves forward.
Payment type
Interest-only
payment structure during the construction phase
Common assumption
55-65%
common average balance used across a full build
Changes over time
Monthly
interest changes as more of the loan gets drawn
Use the calculator
See the build-phase interest while the loan is still in motion.
Start with the total loan amount, your rate, how much has already been drawn, and the build length. The goal is not a perfect penny-level forecast. It is to understand the monthly pressure and the likely total interest exposure while the build is underway.
Interest calculator
Estimate what the build-phase interest is really asking of you.
Construction loans are interest-only during the build, so the payment grows as more of the loan gets drawn. Use this to estimate the current monthly cost and the likely total across the build.
Loan picture
Start with the core loan numbers.
Build-wide estimate
Estimate the average outstanding balance.
Current monthly interest
$1,719/mo
Based on $275,000 currently drawn at 7.5% annual interest.
Average monthly interest
$2,438
Estimated total build-phase interest
$29,250
At full balance
$4,062/mo
This is the highest interest-only payment you may see if the full loan amount is drawn before conversion.
Average balance assumed
$390,000
Uses 60% of the full loan balance across about 12 months.
What this is useful for
Budget the middle
The draw schedule tells you when money goes out. This tells you what those draws are likely to cost while the build is still underway.
What changes the number most
- How quickly draws go out. Faster draw usage usually means the interest-only payment climbs earlier.
- The interest rate. Even a modest rate change moves both the monthly payment and the total build-phase interest.
- How long the build lasts. Interest is time-sensitive, so schedule overruns turn into cost overruns too.
- How much of the loan is actually used. A loan approved for one number may not end up fully drawn, depending on scope and cash contributions.
Why people underestimate construction interest
Many owner-builders know the loan is interest-only during construction, but they still picture one flat monthly number. That is usually not how it works. The interest amount is tied to the running drawn balance, so the payment evolves across the project instead of staying constant.
The result is that the middle and back half of the build often feel more expensive than expected, especially if the project runs longer or if change orders push more dollars into the loan than originally planned.
How this differs from the finished mortgage payment
Construction interest is a temporary build-phase cost. After the home is complete, the loan typically converts into a permanent mortgage with principal and interest amortized over many years. That finished-home payment is a separate number and is often a surprise if nobody has modeled it early.