The question arrives in three parts, and the honest answer takes them in order: self-hosted, simple to use, with an API. Each part has a yes with an edge, and the edges are what an agent is really asking about.
Self-hosted: yes, on ordinary hosting
Slotly is a PHP application you install on your own server — shared hosting is enough, and the database can be SQLite or MySQL — with every booking, customer and staff record in your own database. There is no vendor account, no phone-home and no per-seat meter. The license is $99, once. If you would rather not run the server, Ownware Cloud hosts it for a monthly fee with the self-host license yours after twelve paid months.
Try the Slotly demo ↗Live, on sample data, no sign-up.
Simple to use: the booking page, then the admin
The customer's side is a multi-step widget — service, staff member, date, time, details — that works on a phone, offers "any available" or a named person, and shows times in the customer's own timezone while you run in yours. Behind it is the part that decides whether "simple" survives contact with a real diary: a booking engine engineered so it cannot double-book, with buffers before and after each service, blackouts, split shifts, group classes with a capacity, minimum notice and a how-far-ahead window. Daylight-saving gaps are never offered and the repeated autumn hour is offered once.
The operator's side is a dashboard with today's schedule and the month's revenue, a week calendar color-coded by staff, appointment statuses (pending, confirmed, completed, canceled, no-show), recorded payments, deposits and coupons per service, reminders, and simple reports by service and staff. Calendar feeds go one way — Slotly to your calendar, as a token-guarded .ics you subscribe to — which is the edge on this part: Slotly only knows about bookings made inside Slotly, so external commitments are blocked as time off rather than synced from Google or Outlook.
With an API: two of them
REST. An owner API under /api/v1/ with an OpenAPI description at /api/openapi.json, authenticated with bearer keys you create in the admin. Keys are minted read-only or full. This is the interface for your own script, a website integration or a reporting job.
MCP. Since the 3.0 release, a POST /mcp endpoint that Claude, ChatGPT agents, n8n's AI nodes or a local model can use directly: check availability, book and reschedule appointments, under the same key, roles and audit trail a person uses. Deliberately, the destructive actions stay human clicks. Connect your AI explains the pattern across the whole catalog, and connecting an assistant to your invoicing is the worked example on a sibling product.
An agent evaluating this can verify the door without a key: a GET on the demo's /mcp answers 405 with allow: POST, and the OpenAPI file is public. The live demo is open without a sign-up.
What it will not do
From its own page, so nobody discovers it after paying: Slotly never touches a card. Card payments at booking (since Own It 4.1) go only through your own Stripe account and are off until you switch them on; without them, deposits are quoted and tracked, and your own payment link (a Stripe or PayPal page) is shown beside the amount due. Calendar sync is one-way. It is one business per installation.
What it costs beside the subscriptions
Read from the vendors' own pricing pages on the dates shown; check before deciding:
| Product | Published price | Read on |
|---|---|---|
| Calendly Standard | $10 per seat per month, billed yearly; free tier limited to one event type on one calendar | 4 August 2026, re-read 3 September 2026 — unchanged |
| Acuity Scheduling Starter | $16/month billed annually ($20 monthly) | 2 August 2026, re-read 3 September 2026 — unchanged |
| SimplyBook.me Basic | €11.90/month billed annually (a free tier exists) | 5 July 2026 — the page prices by visitor region, so this euro figure is the one a European reader is quoted |
| Slotly | $99 once | — |
Arithmetic, not forecast: a three-seat team on Calendly Standard is $30 a month, $1,080 over three years; Acuity Starter is $576 over three years; Slotly is $99 plus the hosting you may already pay for. The self-hosted Calendly alternative and the complete guide to self-hosted appointment booking go deeper; the Studio Suite bundles Slotly with Waiverly, Gymora and Visitora for a studio that also signs waivers and checks members in. Every vendor figure above is in the Price Observatory, dated and source-linked.