Thanks to everyone who answered. It looks like the average daily balance is how many credit cards do it, but I'm confused as to how this interacts with the grace period. I'm quoting what my credit card company says, but am not sure I understand it:
INTEREST CHARGE: Method of Computing Amount Subject to Interest: We
calculate the periodic rate or interest portion of the INTEREST CHARGE
by multiplying the applicable Daily Periodic Rate ("DPR") by the
Average Daily Balance ("ADB") (including new transactions) of the
Purchase, Advance and Balance Transfer categories subject to interest,
and then adding together the resulting interest from each category. We
determine the ADB separately for the Purchases, Advances and Balance
Transfer categories. To get the ADB in each category, we add together
the daily balances in those categories for the billing cycle and
divide the result by the number of days in the billing cycle. We
determine the daily balances each day by taking the beginning balance
of those Account categories (including any billed but unpaid interest,
fees, credit insurance and other charges), adding any new interest,
fees, and charges, and subtracting any payments or credits applied
against your Account balances that day. We add a Purchase, Advance or
Balance Transfer to the appropriate balances for those categories on
the later of the transaction date or the first day of the statement
period. Billed but unpaid interest on Purchases, Advances and Balance
Transfers is added to the appropriate balances for those categories
each month on the statement date. Billed but unpaid Advance
Transaction Fees are added to the Advance balance of your Account on
the date they are charged to your Account. Any billed but unpaid fees
on Purchases, credit insurance charges, and other charges are added to
the Purchase balance of the Account on the date they are charged to
the Account. Billed but unpaid fees on Balance Transfers are added to
the Balance Transfer balance of the Account on the date they are
charged to the Account. In other words, billed and unpaid interest,
fees, and charges will be included in the ADB of your Account that
accrues interest and will reduce the amount of credit available to
you. Credit insurance charges are not included in the ADB calculation
for Purchases until the first day of the billing cycle following the
date the credit insurance premium is charged to the Account. Prior
statement balances subject to an interest-free period that have been
paid on or before the payment due date in the current billing cycle
are not included in the ADB calculation.