# Time slot: what is a bookable block in a calendar?

> Time slots explained: how they differ from availability, free and busy states in iCalendar, a slots-per-day calculation and common scheduling-grid mistakes.

Updated on October 15, 2026 · https://app.pilotiko.com/en/glossary/time-slot

A time slot is a specific block of time, with a start and an end, during which a person or resource can deliver a service. In a booking tool, slots are carved out of availability according to the service length, buffers and existing appointments; once a customer reserves a slot, it becomes an appointment and disappears from the list.

## In short
- Availability = the general window; slot = the bookable unit; appointment = a slot that's taken.
- The iCalendar standard distinguishes free, busy, unavailable and tentatively busy time.
- Service length plus buffer decides how many slots fit in a day.
- The grid step (every 15, 30 or 60 minutes) changes which start times are offered.

## How is a time slot different from availability and an appointment?

The three words get used interchangeably, but they mean different things. Availability is the broad window when someone is willing to work, say Monday to Friday from 8 a.m. to 4 p.m. A slot is a piece of that window long enough for a given service. An appointment is a slot a customer has claimed.

That distinction explains why two businesses with identical opening hours can show very different choices online. A shop selling 30-minute services splits its day into far more slots than one whose visits run two hours, and every new booking removes slots for everyone involved in that service.

| Concept | What it describes | Example |
|---|---|---|
| Availability | The recurring hours someone can work | Monday to Friday, 8 a.m. to 4 p.m. |
| Time slot | A specific open block long enough for one service | Tuesday 10:00 to 10:50 |
| Appointment | A slot booked by a customer | Tuesday 10:00, facial, Ms. Tremblay |
| Buffer | Protected time before or after an appointment | 10 minutes of clean-up after each treatment |

Pilotiko working definitions; the matching technical terms are covered below.

## How do calendar systems describe a free slot?

The iCalendar standard, published by the IETF as RFC 5545, defines an FBTYPE parameter to label a period. FREE means the interval is free for scheduling; BUSY means one or more events occupy it; BUSY-UNAVAILABLE means it is busy and cannot be scheduled; BUSY-TENTATIVE means tentative events occupy it [1]. If nothing is specified, the default is BUSY, and software must treat any value it doesn't recognize as BUSY [1]. In other words, when in doubt, a block counts as taken.

A later standard, RFC 7953, starts from a very practical observation: people have regular periods when they're available, such as office hours from 9 a.m. to 5 p.m. on weekdays, yet iCalendar's VFREEBUSY component only represents fixed busy periods, not repeating patterns [2]. It therefore adds a component for publishing recurring availability so it's taken into account in free-busy lookups [2]. That is precisely the step from availability to slots described above.

## How many slots fit in a day?

**Example: a fictional beauty clinic in Rimouski with one aesthetician**

- Daily availability (9 a.m. to 5 p.m.) : 480 min
- − lunch break : 30 min
- = bookable time : 450 min
- Length of a facial : 50 min
- + clean-up buffer after each treatment : 10 min
- = slot length : 60 min
- 450 ÷ 60 : 7.5
- **= At most 7 full facials a day. The leftover half hour can't hold another one, though it could take a 20-minute service if the tool offers it.**

## Why does the grid step change the times customers see?

The number of slots isn't the whole story; you also decide when they can start. With a 60-minute grid, the clinic above offers 9:00, 10:00, 11:00 and so on. With a 15-minute grid, a customer can also pick 9:15 or 9:45, which means more choice but can leave 15- or 30-minute gaps between treatments that nobody can fill.

Common tools expose these settings under other names. Google Calendar, for instance, lets you set appointment duration, add buffer time between bookings, cap bookings per day and enforce a minimum lead time, which defaults to 4 hours [3]. Each of these settings adds or removes visible slots without touching your opening hours.

## Which time-slot mistakes should you avoid?

- Skipping the buffer, so slots line up on paper while delays pile up all day.
- Using a grid that's too fine for long services and creating gaps nobody can book.
- Treating a tentative event as free, when iCalendar counts it as busy.
- Showing one person's slots for a service that needs two people.
- Leaving holidays and time off open so customers book into empty air.
- Confusing slot and duration: a 60-minute slot may hold 50 minutes of service and 10 of buffer.

## How does Pilotiko calculate open slots?

Real open slots are available in Pilotiko and feed its [online booking](https://app.pilotiko.com/en/glossary/online-booking). A slot is offered only if the person involved is free, based on their working hours, time off, closed days and existing appointments, buffers included. Times are computed on a fixed grid set for the business, so two people's slots always line up, and daylight saving changes are handled. When an appointment is saved, conflicts are blocked. The related calendar tools are covered under [scheduling](https://app.pilotiko.com/en/product/booking-and-scheduling) and [dispatch](https://app.pilotiko.com/en/glossary/dispatch).

## Frequently asked questions

### Is a time slot the same as an appointment?

No. A slot is an open block that could be booked; an appointment is a slot someone has claimed. Once booked, the slot disappears from what other customers see.

### What is a tentative slot?

It's a block held by an event that isn't confirmed yet. iCalendar marks it BUSY-TENTATIVE: it isn't free, even though the booking might still fall through.

### How much buffer should I leave between appointments?

There's no universal rule; it depends on clean-up, paperwork or travel between customers. Track a few real days, then set the buffer to the time you usually lose.

### Why do customers see fewer slots than my opening hours suggest?

Because the tool removes existing appointments, buffers, breaks and the minimum lead time. Any gap too short for the chosen service is hidden as well.

### Can one slot involve several people?

Yes, when a service needs two workers, or one worker plus a piece of equipment. The slot is only valid if every required resource is free at the same time.

## Sources

1. IETF, « RFC 5545 : Internet Calendaring and Scheduling Core Object Specification (iCalendar) », https://www.rfc-editor.org/rfc/rfc5545 (accessed on 2026-10-15)
2. IETF, « RFC 7953 : Calendar Availability », https://www.rfc-editor.org/rfc/rfc7953 (accessed on 2026-10-15)
3. Google Calendar Help, « Create an appointment schedule », https://support.google.com/calendar/answer/10729749?hl=en (accessed on 2026-10-15)