A free interest during construction calculator, built by Finwiserr to compute the interest capitalised on drawn debt through a project’s construction phase — the single most fiddly calculation in project finance, isolated in one clean Excel tool.

In project finance, debt drawn during construction accrues interest before the asset earns a penny. That interest is not paid in cash — it is capitalised, rolled into the loan balance and added to the total project cost. The result is a genuinely awkward calculation, because interest during construction depends on the debt balance, which depends on the funding requirement, which in turn depends on the interest during construction. It is circular by nature.

This interest during construction calculator resolves that circularity for you. Enter your construction cost profile, the drawdown schedule and the interest rate, and the tool computes the capitalised interest and the total funded cost — using a VBA routine to iterate through the circular reference rather than leaving you to wrestle with Excel’s iterative calculation settings.

It handles the related financing costs in the same pass — arrangement fees and commitment fees — since these sit alongside IDC in the uses of funds and share the same circular dependency on total project cost.

Why Interest During Construction Matters

IDC is not a rounding error. On a large infrastructure project with a multi-year build, capitalised interest can add a substantial sum to the total financing requirement — which in turn raises the debt that has to be serviced once the asset is operational. Get it wrong and every downstream number, from gearing to DSCR to equity IRR, is wrong with it.

The amount depends on three things this interest during construction calculator lets you flex directly: how long the construction period runs, how the debt is drawn down across it, and the interest rate applied. Debt drawn late in construction capitalises less interest than debt drawn early — and seeing that effect clearly is exactly what the tool is for.

What You Control

• Construction cost profile — the spend to be funded across the build
• Drawdown schedule — how and when debt is drawn against that spend
• Interest rate — the rate applied to the outstanding balance
• Arrangement and commitment fees — the financing costs that sit alongside IDC
• Construction period — the duration over which interest capitalises

Who This Tool Is For

• Project finance analysts who need a quick, reliable IDC figure without building it from scratch
• Developers and sponsors sizing the true funding requirement of a project
• Students and those learning project finance who want to see how IDC circularity is actually solved
• Anyone building a model who wants to sanity-check the IDC in a larger build

For the full picture, read an overview of capitalised interest. Explore our PPP and project finance templates, or talk to us about project finance advisory services.