It's easy to confuse three settings that all affect how many time slots show up on a booking page: how long a service takes, how much buffer sits around it, and the booking interval itself. The first two live on individual services. The booking interval is different -- it's a single, business-wide setting that controls how finely spaced your start times are, and getting it wrong doesn't break anything, it just quietly makes your calendar look either cluttered or too restrictive.
What it actually does
The booking interval decides where slot start times can fall, independent of how long any given service takes. A 15-minute interval means available times show up as 9:00, 9:15, 9:30, 9:45, and so on, regardless of whether the service booked at 9:00 takes 30 minutes or 90. A 30-minute interval halves that density -- 9:00, 9:30, 10:00 -- even for the exact same services.
This is a genuinely different lever from service duration or buffer time. Shortening a service's duration lets you fit more of that specific service into a day. Tightening the booking interval changes how many possible starting points exist across your whole calendar, for every service at once.
Why finer isn't automatically better
A 15-minute interval sounds like it should always be the right default -- more options for clients, more flexibility. In practice it has a real cost: it fills a booking page with a wall of similar-looking time buttons, many of which are functionally identical from a scheduling standpoint (a service that takes an hour doesn't actually get any easier to book by offering it every 15 minutes instead of every 30). For a business where appointments genuinely take an hour or more, a dense 15-minute interval mostly just adds visual noise a client has to scroll past.
Coarser intervals (30 or 60 minutes) tend to fit businesses whose services are longer and less variable in length -- the extra granularity wasn't adding real choice, just more buttons.
When finer actually matters
The opposite is true for businesses whose services are genuinely short and variable -- a quick walk-in-style service, a call, a short consultation. Here, a coarse 30 or 60-minute interval can waste real bookable time: if your service only takes 15 minutes but the interval only offers hourly starting points, you're leaving three unused 15-minute windows an hour that a finer interval would have exposed.
The "once per day" special case
At the far end, an interval of 1440 minutes (24 hours) does something qualitatively different from just "coarse" -- it turns the whole day into a single bookable unit, the right model for a venue, equipment rental, or any service where only one booking per day makes sense regardless of how long it technically runs. The booking page reflects this directly: instead of a specific time, clients see a "Book this day" option, since an exact clock time isn't meaningful when the whole day is the actual unit being reserved.
Picking a starting point
A reasonable default: set the booking interval close to your shortest common service's duration, not your longest. Services only need a divisor of their own duration to book sensibly -- a 60-minute service books fine on a 15-minute interval -- but a 15-minute service never gets properly exposed on anything coarser than its own length. If every service you offer is an hour or longer, there's little reason to keep the interval finer than 30 minutes; if even one service is short, the interval needs to match it.