How ETA Works

PortReckon estimates a vessel's transit time and arrival from three inputs: the sea distance between your ports, the ship's speed, and any slowdown or delay you add. This page explains how that estimate is built, works through a concrete example, and shows what the numbers do and do not mean.

Distance

The route follows real shipping lanes between your origin, an optional waypoint, and the destination, and its length is measured in nautical miles (nmi). One nautical mile equals 1.852 km, so the distance is also shown in kilometres. If no sea lane is found, the tool falls back to a straight-line (great-circle) estimate and marks the result as approximate. Because the route bends around land, this sea distance is normally longer than the straight-line distance between the same two ports - see Major Sea Routes for why.

Speed

Speed is entered in knots, where one knot is one nautical mile per hour. Because a nautical mile is 1.852 km, the app shows a live km/h value next to the field. The default planning speed is 16 knots, a typical figure for a loaded merchant vessel; change it to match the ship or service you are modelling.

The core formula

At its simplest, transit time is distance divided by speed, plus any fixed delay:

transit hours = distance (nmi) / effective speed (knots) + fixed delay (hours)

The effective speed is your entered speed reduced by the total slowdown percentage:

effective speed = entered speed × (1 - total slowdown % / 100)

So a slower ship, or a larger slowdown, takes proportionally longer. A fixed delay is then added on top as a flat number of hours.

Slowdown and delay factors

Real voyages rarely run at full speed the whole way. The tool offers preset factors that each add to your slowdown percentage:

You can also set a manual slowdown percentage and a fixed delay in hours (for example, time spent waiting for a berth). These combine into the effective speed and the final ETA.

A worked example

Suppose the route is 5,000 nmi and you leave the speed at the default 16 knots with no slowdown. The transit time is 5,000 ÷ 16 = 312.5 hours, or about 13.0 days.

Now turn on the Weather Impact preset (+7%). The effective speed drops to 16 × (1 − 7/100) = 14.88 knots, so the same 5,000 nmi now takes about 14.0 days. Add a fixed delay - say 12 hours waiting for a berth - and that half-day is simply added to the total.

Choosing a realistic speed

Distance is fixed by geography, but the ETA is only as realistic as the speed you enter. Large container ships often plan around 16-22 knots, bulk carriers and tankers frequently slower, and "slow steaming" to save fuel can bring speeds down further. If you are comparing two routes, keep the speed the same on both so the difference reflects distance, not assumptions.

What the result means

The output is an estimate for planning and comparison, useful for weighing one route or speed against another. It is not a guaranteed schedule, and it does not account for live weather, currents, traffic, or port operations. For that reason it is best read as "roughly how long", and as a way to see how distance, speed, and delays trade off against each other.

Routes are for planning/visualization only and are not certified for navigation.

This website is provided "as is", for general information and planning only, with no warranty of accuracy, completeness, or fitness for any purpose. To the fullest extent permitted by law, the operators of this website accept no responsibility or liability for any loss, damage, cost, or decision arising from use of the site or reliance on its routes, distances, or ETA estimates. Use at your own risk.