Cwm Mynach via Diffwys and Crib-y-rhiw up Y Llethr: route map, distance and time
756 m · Snowdonia
Y Llethr is the 756 m high point of the Rhinogydd in southern Snowdonia, a grassy summit set among some of the roughest ground in Wales.
A hard mountain walk to the summit from Cwm Mynach public forest path.
Cwm Mynach via Diffwys and Crib-y-rhiw starts at about 193 m and gains 675 m over 7.0 km to 739 m on the track. The first 2 km are the easy part, averaging 7%. The steepest kilometre is the one ending at 4 km, which climbs 215 m at an average of 22%, with a 500 m stretch from 2.9 km at about 33%. The final kilometre eases to the top. About 14% of the way is steeper than 1 in 5, so expect hands-on-knees going and a slow descent.
Cwm Mynach via Diffwys and Crib-y-rhiw at a glance
| Distance | 7.0 km one way, to the top |
|---|---|
| Ascent | 720 m |
| Climb on the track | 675 m up, 132 m down on the way |
| Start and top of the track | 194 m to 741 m (summit 756 m) |
| Steepest 500 m | About 33%, from 2.9 km |
| Steeper than 1 in 5 | 14% of the way |
| Time up | 2h 35m by Naismith's rule |
| There and back | 4h 0m, down the same way |
| Grade | Hard |
| Start | Cwm Mynach public forest path |
| Start grid reference | SH 683 218 |
| Summit grid reference | SH 661 257 |
| Parking | No checked car park for this start; find lawful parking nearby |
Cwm Mynach via Diffwys and Crib-y-rhiw map
The track from the start (green) to the summit (blue), with a marker at each kilometre.
Height profile and getting to the start
Download the Cwm Mynach via Diffwys and Crib-y-rhiw GPX
Ascent only: plan your way down. A planning track, not checked on the ground; it doesn't show safe scrambling moves. Track data © OpenStreetMap contributors (ODbL), heights from a terrain model.
Cwm Mynach via Diffwys and Crib-y-rhiw kilometre by kilometre
Height, climb and average gradient for each kilometre of the track, with a grid reference at the end of each one to check your position on the map.
| At | Height | Climb | Gradient | Grid ref |
|---|---|---|---|---|
| 1 km | 259 m | +66 m | 7% | SH 682 227 |
| 2 km | 332 m | +73 m | 7% | SH 675 233 |
| 3 km | 501 m | +169 m | 17% | SH 667 234 |
| 4 km | 717 m | +215 m | 22% steep | SH 662 236 |
| 5 km | 653 m | -64 m | -6% | SH 666 239 |
| 6 km | 647 m | -6 m | -1% | SH 663 248 |
| 7 km | 739 m | +92 m | 9% | SH 661 257 |
How long Cwm Mynach via Diffwys and Crib-y-rhiw takes
| Pace | To the top | There and back |
|---|---|---|
| Brisk 6 km/h, 8 min per 100 m up | 2h 10m | 3h 20m |
| Steady (Naismith) 5 km/h, 10 min per 100 m up | 2h 35m | 4h 0m |
| Relaxed 4 km/h, 12 min per 100 m up | 3h 10m | 4h 55m |
Times leave out stops. Coming down the same way, the table counts the ascent once; steep descents on rough ground can be as slow as the climb.
- Today: sunrise 07:49, sunset 18:12. Start by 13:12 to be down before dark, allowing 4h 0m for the walk and an hour for stops.
- Tomorrow: sunrise 07:51, sunset 18:10. Start by 13:10 to be down before dark, allowing 4h 0m for the walk and an hour for stops.
Y Llethr summit weather for this route
-
Marginal Today Monday 19 October
Moderate drizzle, up to 7°C at the summit, feeling as cold as -2°C, summit wind 39 mph gusting 49 mph, 49% chance of rain. Rated Marginal. Sunrise 07:49, sunset 18:12.
-
Good Tomorrow Tuesday 20 October
Fog, up to 9°C at the summit, feeling as cold as -4°C, summit wind 24 mph gusting 30 mph, 57% chance of rain. Rated Good. Sunrise 07:51, sunset 18:10.
Best day of the next seven for Cwm Mynach via Diffwys and Crib-y-rhiw: Tuesday 20 October, with summit gusts around 30 mph and a 57% chance of rain.
Hour-by-hour Y Llethr forecast
Before you go, also check: MWIS Snowdonia forecast · Met Office mountain forecast.
What to take today
- An insulated jacket, hat and warm gloves: it feels like -2°C on top.
- Waterproof jacket and overtrousers: 49% chance of rain.
- A windproof layer, and poles for balance in gusts to 49 mph.
- Map and compass, and the skills to use them. A phone is a backup, not a plan.
- A head torch, food, water, a charged phone and a small first aid kit.
About Y Llethr
Y Llethr is the 756 m high point of the Rhinogydd in southern Snowdonia, a grassy summit set among some of the roughest ground in Wales. More about Y Llethr, with the live forecast.
On the lists: Hewitt, Nuttall, Marilyn.
Extend the walk: tops near the summit
- Crib-y-rhiw (681 m), 0.9 km south
- Rhinog Fach (712 m), 1.3 km north
- Diffwys (750 m), 2.4 km south
Challenges that include Y Llethr
More walks from Cwm Mynach public forest path
Routes in Peakify's guides that start within 500 m of this one, for another day from the same car park.
- Cwm Mynach eastern approach up Diffwys
Other routes up Y Llethr
| Route | Distance | Ascent | Time up | Grade |
|---|---|---|---|---|
| Cwm Nantcol western approach | 4.8 km one way | 580 m | 1h 55m | Hard |
| Graigddu-isaf via Rhinog Fach | 6.6 km one way | 660 m | 2h 25m | Hard |
In an emergency
Call 999, ask for the police and then mountain rescue. Give your location (the start is at SH 683 218, the summit at SH 661 257), what happened and how many are in your group. If you have no signal for a call, a text to 999 works once you have registered your phone with emergencySMS: text "register" to 999 before you go.
Cwm Mynach via Diffwys and Crib-y-rhiw: questions and answers
How far is Cwm Mynach via Diffwys and Crib-y-rhiw?
Cwm Mynach via Diffwys and Crib-y-rhiw is 7.0 km one way to the summit of Y Llethr, with 720 m of ascent. There and back by the same way is about 14.0 km.
How long does Cwm Mynach via Diffwys and Crib-y-rhiw take?
About 2h 35m to the summit of Y Llethr and 4h 0m there and back by Naismith's rule (5 km/h plus 10 minutes per 100 m of ascent), before stops. A relaxed group takes nearer 4h 55m for the round trip.
How steep is Cwm Mynach via Diffwys and Crib-y-rhiw?
The steepest 500 m averages about 33%, starting 2.9 km from the start, and 14% of the way is steeper than 1 in 5. Cwm Mynach via Diffwys and Crib-y-rhiw climbs 675 m in all on the track.
Where do you park for Cwm Mynach via Diffwys and Crib-y-rhiw?
Cwm Mynach via Diffwys and Crib-y-rhiw starts from Cwm Mynach public forest path. The grid reference is SH 683 218. No checked car park for this start; find lawful parking nearby
Is Cwm Mynach via Diffwys and Crib-y-rhiw suitable for beginners?
Not as a first mountain. Peakify grades it Hard: steep, rough ground where a slip matters. Build up on an easier route first.
What time should I start Cwm Mynach via Diffwys and Crib-y-rhiw today?
By 13:12 at the latest. Sunset on Y Llethr today is 18:12, Cwm Mynach via Diffwys and Crib-y-rhiw takes about 4h 0m there and back, and that leaves an hour for stops. Start earlier in winter or if the forecast turns.
Is there a GPX for Cwm Mynach via Diffwys and Crib-y-rhiw?
Yes. Download it from this page. It is an ascent-only planning track from the start to the summit, plotted on OpenStreetMap paths and not checked on the ground.