Late fees are only enforceable if disclosed in your contract or invoice terms before the work began, and some states cap the maximum rate, check your local rules.
Late fees compound the way interest does: a monthly rate applied to the unpaid balance for each full month (or partial month, depending on your terms) it stays overdue. The calculator applies your stated rate against the invoice total for the number of late periods you enter.
State laws cap allowable late fee rates in some places, and most freelance contracts use 1-2% per month (roughly 12-24% annualized), which is enough to motivate prompt payment without reading as punitive.
A late fee only changes client behavior if it's stated on the invoice and in the contract before the work starts, applied consistently rather than selectively waived, and paired with clear payment terms (net 15 or net 30, not "whenever"). Silently absorbing late payments trains clients that your deadlines are optional; charging the fee, even a modest one, resets that expectation.
Generally yes if it's disclosed in the contract or on the invoice terms before the work starts, though some states cap the maximum allowable rate: check local rules before setting yours.
Apply it automatically and note it clearly on the revised invoice; asking permission defeats the purpose, since a client who's already late is unlikely to volunteer for a fee.
Many freelancers waive the first occurrence for an otherwise reliable client as goodwill, but state the fee on the invoice regardless, so the expectation is set for future invoices.