By Ryan

Date

Start time UK time (Europe/London)

OS grid reference Either a grid reference or latitude and longitude

Latitude Decimal degrees north

Longitude Decimal degrees; west is negative

Route distance (km)

Total ascent (m) Sum of all climbing on the route

Fitness Very fit (regular hill days)Fit (frequent walker)AverageOccasional walkerNew to hillwalking

Check daylight

How the margin is computed

Your walking time is estimated from distance, ascent and fitness using Naismith's rule with standard corrections, then added to your start time to give an estimated finish. Sunset and twilight times are computed for your exact location and date from standard solar equations (accurate to a minute or two). The comparison produces one of three flags:

• Green — your estimated finish is at least 90 minutes before sunset. That buffer absorbs a slow party, a longer lunch or a navigation error.
• Amber — you are estimated to finish within 90 minutes of sunset, or in the twilight between sunset and civil dusk. Carry a headtorch and know your escape routes.
• Red — your estimated finish is after civil dusk. You would be walking in darkness; start earlier, shorten the route, or plan deliberately for night navigation.

These are planning estimates, not guarantees. A formula cannot see the weather, the ground under your feet or how your group is moving on the day.

Why civil twilight matters

Civil twilight is the period when the sun sits between the horizon and six degrees below it. In open country under a clear sky there is usually still enough light to walk without a torch; once civil dusk passes, the light goes quickly. In the UK civil twilight typically lasts around 30–45 minutes after sunset, a little longer in midsummer and at northern latitudes. It is a useful final buffer — but treat it as one, not as part of the plan.

Winter and summer daylight in UK hills

The UK's daylight swing is large and grows the further north you go. Around the December solstice the Scottish Highlands see roughly seven hours of daylight, with the sun low even at midday; southern England manages only slightly more. In late June the same glens get around eighteen hours, with usable twilight stretching later still. A route that fits comfortably into a June day can be a serious undertaking in December — always check the date, not just the distance.

Usable light is less than official daylight

Sunset times assume a flat horizon and a clear sky. On the hill you rarely get either: heavy cloud can dim the last hour of the day dramatically, and deep valleys, north-facing corries, forestry and steep ground all fall into shadow well before the official times. Descending rough ground in half-light is slow and turns ankle-breaking. Treat the margin this tool shows as a best case, and carry a headtorch (with spare batteries) on every hill day regardless of the flag.