Confirm estimates with a different approach

When estimating, I often start with a bottom up approach. That is, take the individual tasks, estimate each of them, and look at the total for the project. Once that's done, I try to step back and check my estimate with a top-down approach. In that approach, I ask how many people I think I need, for how long, and see what the numbers work out to. I'll keep going, making changes to both estimates, until the numbers converge. It's a nice simple way for me to check for the reasonableness of any estimate. There are other techniques that can be used as well. What's important isn't the technique, it's more that you're doing some sort of double-check before you make the commitment.