There is no point reaching for a calculator that cannot show its working. This one starts from header sizes and reports maximum header span, so you can see how each answer follows from the numbers you entered.
What Is the Door Header Size Calculator?
The door header size calculator turns header sizes into maximum header span, so you can order the right amount of material the first time.
Material estimates are where projects go over budget. Getting the quantities right before the delivery arrives is worth more than any discount on the order.
How to Measure for the Door Header Size
Measure everything in the same unit and take the largest dimension you can reasonably use rather than the average. Formwork, ground conditions and cutting waste all push the real figure above the theoretical one.
Where a plan calls for a specific thickness, check it against local building regulations before you order. The calculator reflects the relationship, not the code.
Allowing for Waste and Errors
Order a little more than the calculation suggests. The gap between the theoretical quantity and what actually arrives on site is normally waste, spillage, over-excavation and uneven grading, and all of it comes out of the same budget.
Write the figures down and compare them with what you actually used on site. That is how you build an estimate you can trust for the next job.
A Worked Example
Working through an example, with header sizes set to 4, the calculator returns maximum header span of 4. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out maximum header span?
Enter header sizes. The calculator then works through the relationship and reports maximum header span.
What does the door header size actually calculate?
It takes header sizes and converts them into maximum header span using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter header sizes?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.