If your household scale is broken, nonexistent or you just want a second opinion, you may be wondering if there's a way you can weigh an object without a scale in a pinch. Though, strictly speaking, there is no way to measure weight without a scale (since any apparatus you put together to measure weight is, technically, a type of scale), the most rudimentary lever-and-fulcrum weight measuring device is one that can be quickly assembled using readily available household objects. For rough weight measurements, this technique can substitute for a professional scale.
Gather some objects whose weights are known to you. Good household objects include hand weights. Or look to your kitchen, where packages of dry food are printed with weight measurements of their contents. You can also use containers of water (a gallon of water weighs 8.35 pounds).
Create a fulcrum for the plank to sit upon. This should be something round, such as a bottle or canister on its side (you'll need to fix it in place on the floor—tape works well for this) or a round-bottomed bowl. Make sure that whatever you use is strong enough to hold the weight of your plank, the object you're measuring and enough measurement objects to equal its weight. The fulcrum must also be high enough to give your plank full clearance of the floor.
Create two measuring pans. This can be something like two aluminum pie tins or two identical plastic bottles with the tops cut off. Place one measuring pan on each end of the plank and attach it so it won't move during the measurement.
Balance your plank with its attached measuring pans on the fulcrum. It should sit with both sides hovering at an equal height above the floor. This may mean the plank is placed evenly with the fulcrum at the middle, but not necessarily.
Place the object you want to measure in one measuring pan, taking care not to slide the plank along the fulcrum. The end you've placed the object on will sink to the ground.
Add objects of known weight to the opposite end of the plank a few at a time. The smaller the increments of weight you have, the more accurate your result will be. Once the plank is hovering evenly again, you've gained equilibrium and you can assess that the object you've weighed is equal to the weight of the known objects you've added.