How does the calculator work?
The calculator takes the inputs you give it and calculates a range of results which help you determine whether or not you are within various towing weight limits. It can be used to figure out what sort of towcar you should buy, or what sort of trailer you can tow.
The calculator cannot however replace a professional trailer weighing operation as that will give you the exact real-world weights as opposed to what the specification sheets say. Here’s a list of trailer weighing operators.
Entering data into the calculator
Yes, it’s unfortunately complex, lots of numbers to go find and enter.
- Actual Trailer Weight – the weight of the trailer, ready to tow. This should be more than the trailer’s tare/kerb weight (which the calculator doesn’t use at present), and less than the trailer’s ATM. You’ll need to measure this on a weighbridge, or estimate.
- ATM – Aggregrate Trailer Mass. The maximum the trailer can weigh, as stamped on its weight placard. The equivalent for the towcar is GVM. Found on the trailer placard (see image below).
- TBM – towball mass. Downforce on the towcar’s towhitch. You will find a guide on the trailer placard, and a maximum on the towcar specification sheet. Also see TBM note below.
- GTM – maximum load on the trailer’s axles. ATM – TBM = GTM. Not used in the calculator.
- Towcar unladen weight (tare or kerb) – the “stock”, unladen weight as on the showroom floor. The definition varies from manufacturer to manufacturer, and tare is slightly different to kerb but there’s no consistent definition of each. Both the trailer and towcar have a tare and/or kerb weight. Consider both weights unladen, but be sure that when comparing vehicles/trailers the same definition is used. Found on carmaker websites, or in owner’s manuals.
- Actual Towcar Weight – the weight of the towcar, ready to tow. This should be more than the towcar’s unladen weight, and hopefully, less than the towcar’s GVM. You’ll need to measure this on a weighbridge, or estimate.
- GVM – Gross Vehicle Mass. The maximum the towcar can legally weigh. ound on carmaker websites, or in owner’s manuals.
- Towcar TBM limit – the maximum TBM the towcar cna handle (see note below). The trailer TBM is the TBM created by the trailer on the towcar, the towcar TBM limit is the maximum TBM the towcar can handle.
- GCM – Gross Combination Mass. The maximum the towcar and trailer can weigh. May be less than GVM + ATM. Found on carmaker websites, or in owner’s manuals.
- Wheelbase – distance between the front and rear axles. Found on carmaker websites, or in owner’s manuals.
- Overhang – distance between the rear axle and towhitch. You’ll need to measure this for yourself.
- Axle loads (front/rear) – maximum load permitted on the front and rear axles, which combined should be greater than, or equal to the GVM. Found on carmaker websites, or in owner’s manuals. This information is sadly not always available.
- % of weight on rear axle – used to calculate the axle loads. Usually 50% for a stock, unladen car, work on 60% for towing, and exclude the effect of TBM as that’s calculated.
Two other terms used:
- Payload (towcar) – difference between tare weight and ATM, amount the towcar can carry. Not required to be measured, this is calculated by the calculator.
- Payload (trailer) – difference between tare weight and ATM, amount the trailer can carry. Not required to the measured, and not currently calculated by the calculator.
Here’s a visual summary:
Here’s an example trailer placard, used for determining ATM, and to give an idea of TBM:
Here’s a vehicle (towcar) placard show maximum towing capacity and towball mass:
And here is a video to explain the weights in brief:
Also read this.
TBM Note – 10%
There is an unfortunate fixation with TBM (towball mass) needing to be 10% of the trailer ATM. This figure is given out as some sort of gospel when in fact there is no science or logic behind it. Many trailer tow perfectly well with 5% TBM, and conversely, some may require 15%. There’s no easy way to determine the optimal TBM, but one way that doesn’t work is to take an abitrary figure and assume it’ll sort all your towing stability problems. I have even seen pro weigh operators suggest to their clients their TBM should be increased from 8 to 10% by putting extra weight on the drawbar…this would increase TBM but very likely reduce stability and certainly increase rear axle load. Now 10% may work for your rig, but don’t make the assumption it’s all you need and a target to aim for. Centralising weight is a much better idea.
Also note that the towcar TBM limit will be the lower of; the car’s TBM rating, the towbar’s TBM rating, and the tow tongue’s TBM rating. So for example a 2012 Ranger PX may have a 335kg TBM maximum, but be fitted with a towbar good for towing 3500kg and 350kg TBM. The maximum TBM remains at 335kg. You always take the lowest figure.
- Combined weight towcar and trailer – simply the actual trailer weight plus actual towcar weight.
- Margin to GCM – the difference between the combined weight and GCM. So if you have a 2500kg car, 2500kg trailer, and GCM of 5500kg that would be 2500+2500=5000 – 5500 = 500kg.
- Margin trailer weight to ATM – your trailer weight should be less than the ATM. Example; ATM is 3000kg, your trailer weighs 2700kg, so 300kg difference. The TBM is included in the trailer weight.
- Margin to max TBM – this is the difference between the towcar’s TBM limit and the trailer’s actual TBM. So for example the towcar has a limit of 250kg, your TBM is 275kg, you’re 25kg over. Watch for European cars which often have TBM limits too low for Australian trailers.
- Rear axle load – the load on the rear axle, and whether it has exceeded the limit or not. Every kilogram of TBM adds around 1.5kg of weight to the rear axle…sounds wrong, but it’s true. Watch this.
- Front axle load – as above, but adding TBM reduces the front axle load, so it’s not normally a limit that’s reached.
- Margin to GVM – how far away you are from your GVM, including effect of TBM.
- Margin to max tow – how far away you are from your maximum tow; for example 2500kg trailer, max tow 3000kg, you’re 500kg away.
- Payload left before reaching a weight limit – how much extra weight you can put in your car when you’re set up to tow. This is a complex calculation; there’s actually three to do; GVM, GCM and rear axle load. Whichever is reached first sets your limit.
- Heaviest trailer you can tow – as above, but heaviest trailer you could tow.
At the bottom of the results are some extra calculations. These are:
- TBM percentage – the towball mass as a percentage of trailer weight. For more on TBM, see above.
- Percentage of trailer weight to towcar weight – towcar weight as a percentage of trailer weight. The heavier the towcar relative to the trailer, the more stable the tow will be. Watch this.
- GCM to GVM + max tow – ideally, the GCM is the same as the sum of the GVM and max tow, so for example GCM 6000, GVM 3000, max tow 3000. If it isn’t, and it is often less, then you’ll often find yourself limited by the GCM rather than the max tow.
- Axle sum limit to GVM – you want the sum of the front and rear axle loads to be more than the GVM, so you have front/rear load flexbility. For example, GVM may be 3000kg, and front axle 1500, rear 1700 total 3200. This is especially important for towers as the rear axle is often the first limitation you run into when determining the heaviest trailer you can tow.
The results are wrong!
Possibly, but before you comment please read this:
- TBM counts towards payload, so if you have say a 3000kg GVM, car weighs 2750kg ready to tow, and TBM is 250kg, then you’re up to 3000kg GVM.
- The heaviest trailer you can tow might be limited by GVM, GCM, maximum braked tow or the rear axle load limit…whichever comes first! So there are multiple calculations done to see which limit is reached first.
- There’s no units marked (kg or lb) as the calculations are the same.
If you do have an improvement suggestion or find an error, please let me know in the comments.
Why can’t I just select my car and trailer and it works it all our for me?
Because every towcar and trailer combination is different. In particular, there’s many different limitations to specific vehicles which one calculator cannot cover without becoming overly complex. For example:
- Some vehicles like Pajeros reduce TBM limits as trailer mass increases.
- Nissan have guidance to reduce GVM by more than the TBM.
- Sometimes the max tow rating changes depending on gearbox and engine.
- Some vehicles require a weight distribution hitch to tow their maximum, or are speed-limited, or require special towing brackets.
What you’ll need to do is get your vehicle and trailer weighed, figure out your load limits, then plug them in.
It is possible to link to a datasource where I can look up much of the data such as GVM and GCM, but that costs significant $ and there’s only so much I can give away for free.
What else do I need to worry about when setting up to tow?
- Your trailer should be as light as possible relative to the towcar. The heavier the trailer, the more the trailer can boss the towcar around.
- More towball mass is not necessarily good – it can prevent trailer sway, but too much brings too much straight-line stability and cornering becomes problematic.
- Trailer sway…a complex subject, explained here.
- Run tyre pressures commensuate with the load. Usually that means a bit more in the rear tyres as those are heavily loaded – watch this to understand rear axle loads. There is actually a reduction in weight on the front tyres.
- Weight distribution hitches? Watch this.
Comment below with suggestions for this page.
I own a Subaru Outback 2018 2.5i Premium and I’m trying to find out the GCM and when I contacted Subaru, they told me that the GCM for my vehicle is the GVM plus ATM. I didn’t think that was correct. Can you please help?
by Alistair Dally
The rear axle weight increase / front axle weight reduction apparently assumes the towball overhang behind the rear axle is half of the wheelbase (1.5 times towball load added to rear axle, 0.5 times subtracted from front axle). I appreciate that is a conservative approach as i gather few vehicles have a longer overhang than that, but given your brilliant graphic that shows so clearly what is happening, perhaps you could provide an option for those that know wheelbase and towball overhang to enter those distances and get more accurate vehicle axle weight changes. Either way, thanks for a useful tool
by Ron Oxley
Excellent advice, excellent towweights calculator – I just plugged in my numbers which I knew to be near the compliance limits (under Evernew heading) – but was pretty sure they were generally Ok and yr info confirmed. However………yr calculator does not specifically seek to address rear axle ratings compliance – unless I missed something (I know your calculator results did mentioned rear axle load @ +495kg)……& it seems to me (with the numbers I’ve done on a few cars now ) that rear axle rating non compliance is proving to be a very big stumbling block if you want to pull a heavy van.
eg landcrusier 200 series – rear axle rating 1950….but by my calcs, my rear axle load is 1986 ! – using a rear axle load factor of 150 (1300/2850* 330 tbm……on a van atm of 3300 )) results in a rear axle load of 330 +150= 480. (Yr calculator indicated a rear axle load with my numbers of 495 so it either used a higher tbm or slightly different rear axle to hitch & wheelbase dimensions…but it doesnt matter anyway). Both numbers result in rear axle rating non compliance (ie 3012 total car weight/2 = 1506 + 480 = 1986 or 1506 + yr number of 495 = 2001.
V interested in yr thoughts. Cheers Ron
by Robert Pepper
I may add rear axle loads to a future version, for the moment if you take the TBM and x 1.5 that’ll be close enough with a little safety margin. You have the correct formula there (WB/overhang) but also allow for hitch length which varies. 4X4 wheelbase is typically 3m or so, overhang around 1.4-1.6 so it works quite well.
by Ron Oxley
Thks- understand now where your rear axle load number comes from. Can I make a final comment. As u know, vastly better than I, there is mass confusion “out there” re car/van weights. In all this confusion, rear axle ratings almost dont rate a mention & the v great majority of non car enthusiasts would not give it a thought! (I fit this category and only stumbled across this issue after hours and hours of reading stuff ……& finally a lot of help from you (yr excellent “Example tow weights” video should be mandatory reading for anyone pulling a van). Anyway my key point is this – the REAR AXLE RATING for the cars most favoured to pull a heavy van (ie one over 3000kgs) becomes perhaps THE MOST CRITICAL FACTOR to evaluate for legal compliance . Two examples (1) Toyota 200 series car with a payload of just 250 kg (eg 2 people = 150kg + 100kg allowance for absolutely everything else ie not that much!) cannot pull a van with an ATM of 3200 or greater without failing toyota’s rear axle rating. (2 ) RAM 1500 Laramie -an even worse example…notwithstanding its longer wheelbase. It has a rear axle rating of only 1770kg (& a kerb mass of 2650) Allowing for a payload of just 150 kg (ie 2 people in the car AND NOTHING ELSE!) it cannot pull a van with an ATM of 2800 kg or heavier without failing its rear axle rating. (My maths for the Laramie is as follows – rear axke load factor 1300/3569 = 0.364 with a tbm of 280 =102 + 280 = rear axle weight due to van download of 382. 382+1400 (2800/2) = 1782kg . Rating 1770).
I’m just shocked with these outcomes- given the marketing associated with these cars. Am I missing something or over-reacting? Cheers Ron.
by Robert Pepper
no, you’re not missing anything. I was the first journalist to alert people to the 3500kg tow issue, now I’m presently alone on rear axle loads. But I have plans to make a noise…it is a problem!!!
On “% of weight on rear axle”.
Is there any way to calculate this based just on vehicle specifications? For example, Everest Trend front axle 1480 rear 1750 with a GVM of 3100, can I take an approximate ratio of 1750/GVM or 1750/(1480+1750)?
How could I determine the ratio without buying the van first?
by Robert Pepper
No accurate way other than a weighbridge or ask owners to record their weights. Usually 4X4s are around 50/50 front/rear. The greater rear axle load doesn’t indicate more weight on the rear axle, it is so the back can be loaded more than the front eg when towing.
I think your Margin to GCM may be incorrect as it calculates using GVM + ATM to get the GCM
RACQ, Without a Hitch and many others, calculate GCM as:
GTM = ATM – tow ball mass
GCM = GTM + GVM
by Robert Pepper
GCM = GTM + GVM ignores TBM, unless it’s included in GVM. Regardless, the result is the same as my method of ATM + GVM, where I include the TBM in ATM.
by Ben Mears
Thanks. I’ll check but I think it is doubling the tow ball weight. I’ll have a look tomorrow
by Max Bowyer
Thank you Robert for the many videos etc that I have viewed. I was led to your calculator and I have questions about the towing legality of a 2015 Colorado 7 (which I have used since new to tow a Traveller Caravan over 40,000 kms).
Entering details for 1) Colorado 7 with just the driver as payload 2) a much lighter caravan than mine (1500kg) into your calculator flags a rear axle overload of 42kg @ 60% weight on axle. By changing it to 58% it just becomes acceptable. Surely the Colorado 7 is not limited to 1500 kg as my van is much heavier. I am happy to email the document I created from your calculator, so that you can highlight where I am making an data entry error. Much appreciated (happy to speak on the phone too 0422206854)
by Robert Pepper
The rear axle load is a function of the TBM, not the total trailer weight. The Colorado can tow much more than 1500kg, but you may find the rear axle load is exceeded early.
by Mark Gregory
Would I be correct, or overly optimistic, in thinking that the towing rating of 3500kg on a vehicle would indicate that the braking system at least is properly specc’d for the 3,500kg nominal load at the rear of the vehicle. (Never mind that the axle load limits may make the tow rating rating fanciful).
by Robert Pepper
Sort of…would rely on trailer brakes. But, no 3500kg ute or wagon should be used to tow 3500kg for a long time, if ever.
by Roger Cripps
So How do you find the correct ball weight for your car
I can change it lots either way up or down