Tow hitch 'S' & 'D' Values Explained

S & D Values Explained

‘S’ and ‘D’ values can be confusing, even for experts! Though a little tricky, it’s important to calculate these values correctly to ensure that you tow safely and in compliance with the law.

Here is our simple explanation of what each value is, how to calculate it and why it is important to get it right.

S Value

What is an ‘S’ value?

Sometimes called the ‘nose weight’, this is the weight that the drawbar of your trailer exerts on your towbar. The ‘S’ value is measured in kg and represents the maximum vertical load which can be applied to the towing jaw/hitch.

Note that the ‘S’ value is not the same as the gross vehicle weight or GVW of the trailer and has no reference to the total weight the towbar is capable of pulling.

All towing brackets must also show the ‘S’ value on their respective manufacturer's label – so this should be relatively simple to find.

Check out our comprehensive range of vehicle tow hitches and accessories HERE.

D Values

What is a ‘D’ value?

The ‘D’ value is a little more complicated. The ‘D’ stands for drawbar and is measured in kN (kilonewtons).

The capacity of your towing equipment will vary depending on the GVW (gross vehicle weight). Your GVW refers to the maximum weight your vehicle can safely operate under and includes everything in it including passengers and load.

The ‘D’ value of a Towing Jaw determines the gross weight combination of the towing vehicle and the trailer that can be used with that particular jaw. In other words, the higher the ‘D’ value, the greater the mass that can be attached to a coupling.

Explore our large range of couplings and coupling spares suitable for a diverse range of manufacturers HERE.


How do we calculate the ‘D’ value?

With the following formula:

D = 9.81 × (T × C) ÷ (T + C)
T = Towing Vehicle Weight - GVW
C = Trailer Weight - GVW
9.81 = Gravity

For example, a towing vehicle of 3500kg GVW combined with a trailer of 3500kg GVW would require a towing hitch with a ‘D’ value of:

9.81 × (3.5 × 3.5) ÷ (3.5 + 3.5) = 17.167 kN


IMPORTANT TO NOTE: It is ILLEGAL to tow a vehicle that exceeds the correct GVW (gross vehicle weight) for your Towing Jaw.

Use our calculator below to work out if you are permitted to tow before you set off.


Use the below tables to calculate the ‘D’ values, the maximum trailer GVW, or the maximum towing vehicle GVW. These tables are based on standard UK trailers up to 3,500kg GVW.


Calculate required hitch "D" value




Calculate maximum trailer GVW




Calculate maximum towing vehicle GVW



Did you know?

The TP22 Dixon-Bate style ball and pin hitch '3500kg' round top pin supplied by TRAILPARTS Ltd is rated to a ‘D’ rating of 20.2kN. This allows the TP22 to be safely and legally fitted to the newer, larger specification LCV's with a GVW of 5000kg whilst still allowing them to tow trailers up to 3500kg GVW.


Trailparts is the best place to get all your spare and replacement parts for your trailers. If you need further expert support, get in contact with one of the friendly team at Trailparts today HERE!

Notwithstanding the above information, the responsibility of ensuring the correct vehicle and trailer combination rests entirely with the user. Please click here for more information about matching the trailer with the towing vehicle (link opens external website in new window). Further information can be found on the National Trailer & Towing Association website.