A project budget with a contingency built in

Enter the contractor's estimate, the contingency share you want set aside, and the costs outside the estimate. The calculator shows the contingency in dollars and the budget in all.

Where to find your numbers

• Estimate. Enter the total on the written estimate you'd accept. It should list the work, the materials, the completion date and the price.
• Contingency. Enter the percentage of the estimate you want held back for surprises, such as damage found behind a wall or a change you decide on partway through.
• Permits, design and other costs. Enter everything the project needs that isn't in the estimate. That can include permit fees, an architect's or designer's charges, materials you're buying yourself, appliances, a storage unit, or meals out while the kitchen is torn up.

What the figures mean

Contingency is the estimate times the percentage you entered. Budget is the estimate, plus the contingency, plus the other costs. It's the figure to plan the project's money around, rather than the estimate alone.

The contingency is worked out on the estimate only. If the other costs are large, or uncertain themselves, add a cushion to that line too.

Comparing estimates

Get more than one estimate, and don't take the lowest bid just because it's lowest. If the estimates differ a lot, ask the contractors why. Run the calculator once for each, with the same contingency and other costs, so the budgets compare like with like. Check that each estimate covers the same work and materials before reading the totals side by side.

Keeping the budget current

Once the work starts, run the figures again whenever something changes. A change you ask for partway through comes out of the contingency, or it raises the budget. When a bill for something outside the estimate arrives, move it into the other costs line. Keep a running total of what's spent or committed, and compare it with the budget.

What it leaves out

The calculator doesn't count the cost of borrowing, if the project is financed, or any change in your property tax afterward. It doesn't spread the budget over the months of the job either. Don't pay the full amount up front, and hold the final payment until the work is finished and you're satisfied with it.