Navigation Tools — the 16 Offline Calculators

Modified on Wed, 20 May at 9:50 PM

KnotWise includes 16 navigation calculators covering everyday passage-making and celestial work. All calculators are available on every plan — including Free — and run fully offline. Access them through Navigation Tools in the left sidebar.

Important: These calculators are navigational aids. Results depend on the accuracy of your inputs. Always cross-check against official charts, almanacs and tide tables before making safety-critical decisions.

How the calculators work

KnotWise provides the calculation model. Every input — sextant readings, tidal differences, deviation values, almanac data — comes from you. The app makes no assumptions about where your numbers originate, and never pre-fills a value from an internal source without your action.

This is deliberate. Navigation decisions remain with the navigator. A calculator that silently injects data the navigator has not verified hides errors rather than preventing them. Every result KnotWise produces can be traced back to a value you entered, with the formula and intermediate steps available on screen.

Practical navigation — 11 calculators

The first cluster covers daily passage-making — bearing, course, tides, compass correction and dead-reckoning fixes.

Calculator	What it computes	Typical input source
Course & Bearing	Rhumb line and great circle course and distance between two positions	Chart or GPS
Compass Correction	Converts between true, magnetic and compass course using variation and deviation	Chart (variation) and deviation card
Speed · Distance · Time	Solves the classic SDT triangle — calculate any value from the other two	Log, GPS, chart
Waypoint & XTE	Bearing and distance to a waypoint, cross-track error and ETA	Chart or chartplotter
Rule of Twelfths	Tidal height at any point between low and high water	Tide table (Reeds, Admiralty)
Light & Visibility	Geographic and nominal range of a navigational light	List of Lights, chart
Current Triangle	True course and speed made good accounting for tide or current	Tidal stream atlas, observation
Leeway	Leeway angle and course correction for wind drift	Observation, wake angle
Secondary Port Tides	Tidal predictions for secondary ports using standard port data	Standard port table, time and height differences
Running Fix	Position fix from two bearings of the same object taken at different times	Hand bearing compass, log
Unit Converter	Nautical and meteorological unit conversions	—

Celestial navigation — 5 calculators

The second cluster handles sight reduction and astronomical position fixing. Bring your sextant.

Calculator	What it computes	Typical input source
Sun Sight Reduction	Full intercept method reduction from sextant reading to position line	Sextant, UTC, assumed position
Star Sight Reduction	Sight reduction for the 57 navigational stars	Sextant, UTC, assumed position
Noon Sight	Latitude from meridian passage of the sun	Sextant at local apparent noon
Compass Check via Azimuth	Compass error from computed and observed azimuth of the sun	Hand bearing compass, UTC, position
Estimated Position	Fix from two to four position lines using the intercept method	Results from preceding sight reductions

What every calculator shares

A few elements work the same way across all 16 tools, so they are worth learning once.

Coordinate format toggle. Switch between DD°MM.mmm and decimal degrees from the entry screen. The setting persists between sessions.
Formula panel. Every calculator carries an expandable panel showing the formula and intermediate values. Useful for cross-checking or for teaching crew.
Source note. Calculators that depend on external reference data carry a short note stating where inputs should come from — for example, Reeds for tides, the deviation card for compass correction.
No automatic persistence. Inputs live in the current session only. Close the page and the values are gone. This is intentional — a calculation is a point-in-time event, not a record. If you want a record, export it.

Saving and exporting results

Every calculator carries two output options on the result screen.

Export PDF — generates a formatted PDF with all inputs, intermediate values, the result and any relevant diagram. The PDF includes your vessel name and last log entry as context, in the same format as other KnotWise PDFs.
Save to Logbook — creates a new log entry pre-filled with a compact summary of the calculation. Useful for documenting a sight reduction, a fix or a tidal calculation as part of the passage record.

Both actions are one tap from the result. Neither requires a paid plan.

Accuracy and limits

The calculators are designed to professional standards, with the following limits documented for each cluster.

Celestial ephemeris. GHA and declination are computed using Meeus algorithms, accurate to better than 0.1 arc minute for any date within 50 years of J2000.0. No internet connection required, ever.
Tidal calculations. The Rule of Twelfths and Secondary Port models produce harmonic approximations. They do not replace official tide tables for pilotage decisions in tidal harbours — verify against Reeds, Admiralty Tide Tables or the local national hydrographic source.
Running fix. The fix is reliable when the crossing angle between bearings is between 30° and 150°. Angles outside that range produce poor fixes — the calculator warns when you cross the threshold.
Source data. Results are only as accurate as the inputs you provide. KnotWise does not validate sextant readings, almanac values or tide table extracts against external sources.

The calculators do not replace charts, almanacs or official references. They run the arithmetic — the observation, the seamanship and the judgement remain yours.

Frequently asked questions

Do I need a subscription?

No. All 16 calculators are available on the Free plan. There is no upgrade path that unlocks additional navigation tools — they are part of the core offering. See knotwise.app/pricing for full plan details.

Do the calculators work offline?

Yes. All 16 run fully offline, including the celestial calculators. The astronomical ephemeris is computed locally — no satellite or data connection required.

Are my inputs saved between sessions?

No, by design. Inputs are kept only for the current session. Closing or reloading the page clears them. To preserve a calculation, use Export PDF or Save to Logbook.

Can I use a calculator result in a log entry?

Yes. Save to Logbook creates a new entry pre-filled with the calculation summary, ready for you to add context — sea state, crew, observations — before saving.

Which references are recommended for input data?

Reeds Nautical Almanac or Admiralty Tide Tables for tides; the Nautisches Jahrbuch or Nautical Almanac for celestial work; your vessel's deviation card for compass corrections; your chart or chartplotter for positions and bearings.