What a payment plan really costs: the 20 lines of Python behind a dental financing calculator

Smile Budget — Wed, 04 Nov 2026 22:43:23 +0000

When someone is quoted a few thousand dollars for dental work, the question that follows is rarely "can I afford this?" It is "what will the monthly payment do to the total?" That is a small piece of arithmetic, and it is a good first project if you are learning Python.

Here is the whole idea in one paragraph: a payment plan is a principal, a rate and a number of months. The monthly payment follows from those three, and the total cost is the monthly payment times the months, plus any fee you pay up front.

The formula

For a loan with a fixed annual rate r and n monthly payments, the payment on principal P is:

def monthly_payment(principal: float, annual_rate: float, months: int) -> float:
    if annual_rate == 0:
        return principal / months
    i = annual_rate / 12
    return principal * i / (1 - (1 + i) ** -months)

The zero-rate branch matters. A lot of dental payment offers are "no interest if paid within the promotional period", and without the special case the function divides by zero.

The part people forget: total cost

def total_cost(principal, annual_rate, months, upfront_fee=0.0):
    pay = monthly_payment(principal, annual_rate, months)
    return round(pay * months + upfront_fee, 2)

Try three offers for the same $3,000 treatment:

offers = {
    "12 mo, 0% promo": (3000, 0.0, 12, 0),
    "24 mo, 14.9%":    (3000, 0.149, 24, 0),
    "36 mo, 9.9% + $75 fee": (3000, 0.099, 36, 75),
}
for name, args in offers.items():
    print(f"{name:24} total ${total_cost(*args):>8,.2f}")

The output makes the point faster than any sentence. The lowest monthly payment is on the longest plan, and the longest plan is not the cheapest.

The edge case that matters

Deferred-interest promotions (sometimes written "no interest if paid in full by...") are not the same as a true 0% loan. If the balance is not cleared by the end date, interest may be charged back to the first day. You can model that with a small check:

def deferred_interest_risk(principal, promo_months, annual_rate, paid_after):
    unpaid = max(0.0, principal - paid_after)
    return round(unpaid * annual_rate * (promo_months / 12), 2)

It is a rough number, not a lender's. The takeaway for the code is to make the promo end date an explicit input rather than burying it in a comment.

Make it a tool

Wrap the functions in a small CLI or a web form, add a column for the deposit, and let the user paste in two or three offers. Sorting by total instead of by monthly payment is the single most useful design choice.

If you would rather see what this looks like as a finished page, I keep a reference version of a dental financing calculator on a small educational site. It does not replace the terms in your own agreement; the numbers above are just arithmetic.

Where this could go next

Add a bi-weekly payment option.
Read offers from a CSV so a family can compare three or four at once.
Show the cash price next to the financed totals, since a pay-in-full discount sometimes changes the answer.