There are two main factor that you should look while selecting the right tongue jack: weight and height capacity jack Weight:: The weight capacity of tongue jack is identified by calculating the tongue weight of your trailer, to understand this the formula is very easy.The rule of thumb for a boat trailer is almost 7 to 8 percent of the weight of your fully loaded trailer.If you have a 7,000 lb loaded trailer you don't need to go with a 7000 lb electric trailer jack, using just its 10% as an example enough that you require a tongue jack which has a capacity of almost 7,000 lb. Height: You need to have the right height for the jack to lift to clear the problem on your vehicle lifting. For height measurement on tongue jack you need to figure out the level. Measure the height from ground to the bottom of the coupler. This measurement must be done when the trailer is on level.