Ask HN: What's the interest rate on technical debt?

It seems we're now stuck with the term "Technical Debt" to describe things needing cleanup. Non-technical people need a number we can use to express just how expensive putting off cleanup will be to make rational decisions we can all live with.

What is your personal estimate of the interest rate we pay on technical debt?

My personal opinion is that it's around 100% per month, that is every hour of refactoring put off for a month doubles in size. That is, it wastes an hour elsewhere, AND still needs to be done.

Or is the interest, just how much more code we have to write to work around the debt?

Also, what units is the denominator in the fraction? Is it code size, functionality, or just calendar time?