When you are estimating the cost to build a prop, you often add a “contingency.” Take 15-25% of the anticipated cost and tack it on to the estimate. So if you predict that a prop will cost $100, a 15% contingency is $15, making your new estimate $115. I recently heard a student ask whether a contingency should be applied to every item in your estimate, or to the estimate as a whole.
To help me think this through, I looked up the definition of contingency. It is a “provision for an unforeseen event or circumstance.”
When you are purchasing your materials, you probably want to buy a little extra. You may mess up a cut, you might measure something wrong, a section might be damaged, or you may have underestimated how much you will need in your original plan. Often, the cost of getting just a little bit more material later is greater than the cost of buying a little extra material at the beginning. Materials are often cheaper in bulk. If you buy them online, it is cheaper to pay shipping once rather than twice, and many material suppliers have minimum order requirements. Even if you can get things locally, the cost of multiple purchases can add up; sure, a single screw may only be 13 cents, but the half hour trip to the hardware store adds several dollars worth of time to its price.
So pad your material needs. When I buy hardware, I buy by the box to make sure I have enough. Sometimes you use more than you originally thought, sometimes you just drop a few screws and can’t find them. When I buy fabric, I round up to the nearest yard (or add a few yards if it is cheap enough). Especially when it is materials I know I can use for future projects, any extra will go onto my shelves and save money down the line.
Not every material or line item will be padded, of course. If your project requires a motor that costs $200, you’re not going to buy two of them, right? I mean, not unless you’re planning for a lengthy open-ended run and your company has the money.
I do not really think of material padding as part of your contingency, because it is not an “unforeseen event”. The contingency is added on top of everything at the end. It is for costs you could not have planned for or for costs that come up because of changing circumstances. “Oh, I need to buy degreaser to clean this steel.” “Oops, I need to buy rags to apply this stain.” “I’ve just been told I need to buy a drop cloth before painting this.” “This prop is heavier than I anticipated and will need handles.” “Now they want to put a light inside this magic wand.”
So pad the amount of materials as needed, than add a contingency to the top of everything. That’s what I find works for me.