There's money to be made in pouring concrete, but it's a lot more complicated than "mud pies for grownups." You'll need to know which mix is right for which jobs, how many yards of concrete you'll need to order and – crucially – how much rebar you'll need to reinforce that concrete so it holds up over time and use. The math isn't especially difficult, but you'll need to master it before you can give your potential clients an accurate quote.
In an ideal situation, you'll quote based on architectural drawings that specify what size of rebar you'll use and what the spacing should be from center to center. In a less formal setting where you're pouring a slab in an existing space without formal plans, you'll have to do a bit more research.
Your best option is to look up your local building code and see the size and spacing of rebar for which it calls. It can also be helpful to ask around and see what other contractors do in similar settings. If there's a best-practices option that goes beyond your local code – using 12-inch spacing instead of 14, for example, or #5 rebar instead of #4 – you may want to show that to your client and explain the benefits in added durability and reliability. You'll need to allow for the expected loads on your slab too. A basement rec room won't need to be as sturdy as the floor of a garage that will hold trucks and bulldozers.
For the sake of easy examples, assume you're pouring a slab that's a simple rectangle of 20 by 30 feet, and you've figured out that you'll need the rebar set at 14 inches on center. Start by multiplying each of those measurements by 12 to get your dimensions in inches, which in this case would be 240 inches by 360. Now, divide each measurement by the 14-inch spacing. For the short side, 240 inches divided by 14 inches gives you 17 lengths of 20-foot rebar plus a bit left over. Add one more length to allow for that extra, giving you a total of 18. For the slab's longer dimension, you'll need 360 inches divided by 14, which gives you 25 plus a bit left over. That means you'll need a total of 26 lengths of 30-foot rebar.
Depending where you live and how much rebar you're ordering, you may not always have the option of ordering it by the individual piece. Instead, you might be ordering by weight. Converting isn't that big of a challenge because most manufacturers and suppliers provide you with the weights of their rebar on their websites. First, add up the total length of rebar you need. In this example, you'd need 18 lengths at 20 feet for a total of 360 linear feet. You'll also need 26 at 30 feet for a total of 780 linear feet. Put those together, and your total order is 1,140 feet of rebar. Now, look up the weight of your rebar. If you're using #5 bars, for example, they come in at just over a pound per foot – 1.043, to be precise. Multiply that by your 1,140 feet, and you'll find you need to order 1,189 pounds of rebar.
If you're going to be pouring concrete a lot, you'll want a quicker way to do all of this. The obvious answer is to find a construction-oriented calculator, which is preprogrammed so you can just enter the dimensions of your slab and your chosen spacing for the bars. You'll find any number of these online or in the form of an app for your phone. You can also look at buying a real, physical calculator that fits in your pocket and can give you your answer in either feet or weight of rebar. Physical calculators have a couple of advantages on the job site. They're usually smaller and more rugged, they're cheaper to replace than your phone if you smash one and you can use one with gloves on or with your hands crusted in mud or concrete.
This is just a simple example, and it leaves out a few real-world considerations. For one thing, code in most jurisdictions calls for the rebar to stop 2 to 3 inches before the edge of the slab. You'll need to adjust your measurements to allow for that. You'll also need to figure out what lengths you want to order. In this example you need 20- and 30-foot lengths, but 30 feet is not a standard size for rebar. It might be more cost effective to order standard 60-foot lengths and cut them in half or to order all 40-foot lengths and cut them onsite to make up both 20-foot lengths and, with suitable degrees of overlap, 30-foot lengths.
The cost of the rebar itself is only part of the picture, of course. Before you draw up a final estimate for your client, you'll also need to take a lot of secondary costs into consideration. You may need supports, or "chairs," to hold your rebar at the right depth within the slab, and you'll definitely need wire to hold the bars together wherever they cross.
If the shape of your slab makes for a lot of bending or cutting, you'll need to allow for the labor costs involved in doing that extra work. Most importantly, you need to allow for waste and off-cuts. If you're making up lengths using shorter pieces, you'll also have to allow for several inches of overlap – whatever's called for in your local building code. Allowing about 10 percent extra for all of this is a good rule of thumb to get you started. If you have an especially complicated shape with which to work, you might have to allow more for wastage.