Tide predictions that work offline, computed right in your browser. No account, no API keys, no tracking.

→ Open the app

Install it to your home screen and it keeps working without a connection.

• Now + next: whether the tide is rising or falling, the current height, and a countdown to the next high or low.
• Scrub the day: drag across the chart — or use the arrow keys — to read the predicted tide at any moment. Times are labelled with the station's timezone, so they're never mistaken for your own clock.
• Pick anywhere: use your location, search for a place or beach, or tap the map. Lunitidal snaps to the nearest tide station and tells you how far away it is and how trustworthy the match is. On a first visit it guesses your region from your IP and starts there (falling back to Benoa, Bali).
• Waves & swell: an optional marine forecast layered on top of the astronomical tide (needs a connection; degrades gracefully offline).
• Make it yours: save favorite spots and set your units (m/ft, km/mi), time format, theme, and chart datum.

1. Open the app. It starts at an approximate location based on your IP (or Benoa, Bali, if that's unavailable), then remembers your last location next time.
2. Tap Change to use your location, search a place/beach/station, or open the map.
3. Read the big readout for the current state; drag the chart to see any time of day; use ‹ › to move between days, or tap the date to jump back to today.
4. Tap ⓘ Source & accuracy to see which station the data comes from, switch the chart datum, and view the underlying harmonic constituents.
5. Tap ☆ to save a spot; manage saved spots and preferences in Settings (⚙).

Predictions are astronomical only — the regular rise and fall driven by the moon and sun. They don't include weather effects (wind, pressure, storm surge), and they come from the nearest tide station, which may be some distance from your exact spot. Treat them as a good guide, not for navigation.

Tide data is from TICON-4 (UHSLC sea-level analysis). Heights can differ from official port tables by a few tenths of a metre because it's an independent analysis, not the official port solution.

The waves & swell card is a separate, online-only layer from Open-Meteo, shown on top of the astronomical tide.

Open-Meteo's wave data comes from numerical wave models that divide the ocean surface into a grid of cells — anywhere from a few kilometres to ~25 km across — and each cell holds one forecast. Crucially, these models only run over water: there's no wave value for land, harbours, or small enclosed bays.

So when you pick a spot, the API snaps your request to the nearest grid cell that actually has data and tells us which cell it used. For a beach, harbour, or bayside point that cell can sit a few kilometres away, in any direction. That's why the card labels the source like:

Open-Meteo · ~2.8 km SSE ⓘ

The number is the distance from your chosen point to the grid cell the forecast came from, and the compass bearing points from you toward that cell (here, the waves describe the water ~2.8 km to the south-southeast). It is not a distance to the shoreline. Tap the ⓘ for the same explanation in-app. If no usable water cell is nearby, the card shows "No marine forecast for this location" instead.

There are no accounts and no server — Lunitidal runs entirely in your browser. Your saved spots and preferences (units, time format, theme, chart datum) live in this browser's local storage on this device only: they don't sync across devices, and clearing the site's data resets them. The only network requests are for optional extras — place search, the marine forecast, map tiles, and a one-time IP-based guess at your starting region — and nothing you do is tracked or sent to us.

• Tide data: TICON-4 / UHSLC via @neaps/tide-database (CC-BY-4.0).
• Tide prediction engine: @neaps/tide-predictor (MIT).
• Place search & marine forecast: Open-Meteo (CC-BY-4.0).
• Approximate IP location: geojs.io.
• Map tiles: OpenFreeMap / OpenStreetMap (ODbL).

Lunitidal is a no-backend, offline-first PWA: Svelte + Vite + TypeScript, tooling via Bun, deployed to GitHub Pages.

• Bun (package manager, script runner, and test runtime)

bun install
bun run dev        # http://localhost:5173

predev/prebuild automatically run build:data first — a build-time script that reads the large @neaps/tide-database package and emits the small static files the browser uses (a slim station index, a bundled Benoa seed, and per-station constituent files). The raw database is never shipped to the client.

bun run dev          # dev server
bun run build        # production build (outputs to dist/)
bun run preview      # serve the production build locally
bun run build:data   # regenerate public/data/* from @neaps/tide-database
bun run test         # unit tests (Vitest)
bun run test:e2e     # end-to-end tests (Playwright)
bun run test:all     # unit + e2e — run this before pushing
bun run check        # type-check (svelte-check)

For e2e you'll need the browser once: bunx playwright install chromium.

scripts/   build-time tooling (station-index extraction, icon generation)
src/
  engine/      pure, framework-agnostic core (prediction, datum, time, units,
               station lookup) — heavily unit-tested
  chart/       pure SVG chart geometry
  sources/     network sources (geocode, reverse-geocode, marine, IP location)
  stores/      Svelte stores (selection, settings, favorites, persistence)
  lib/         shared constants/helpers (e.g. external links)
  components/  UI components
  routes/      Home / Detail / Settings (hash-based routing)
tests/e2e/   Playwright integration tests
public/data/ generated station data (git-ignored)

Unit tests cover the engine and data-layer boundaries; Playwright covers the UI end to end against a production build (so the real service worker and offline paths are exercised). Every push runs the full suite in GitHub Actions, and deploys to GitHub Pages only if it passes. Please run bun run test:all before pushing.

See CLAUDE.md for non-obvious gotchas worth knowing before you dig in.

The application code is MIT-licensed — see LICENSE. The third-party tide data, forecasts, and map tiles it relies on carry their own licences (CC-BY-4.0, ODbL) with attribution requirements; those sources are listed under Credits.