Performance Budget Calculator
Set a page weight budget from a target load time and connection speed, allocate it across scripts, images, fonts and CSS, and see which asset type is consuming the budget.
Last reviewed by the Radiatus Cloud team
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A budget is a number that can be exceeded
"Make the site fast" cannot be violated, so it never is, and pages grow anyway. A budget of 170 KB of JavaScript can be exceeded, and exceeding it is visible in a build check, in a code review and in a conversation. That is the entire mechanism: performance work fails not because people disagree with it but because nothing tells them when they have broken it. Deriving the number from a target load time on a realistic connection makes it defensible rather than arbitrary.
Not all bytes cost the same
A hundred kilobytes of JavaScript costs far more than a hundred kilobytes of image, because the script must be downloaded, parsed, compiled and executed on the main thread while an image is decoded off it and can be lazy loaded entirely. On a mid-range phone the parse and execute cost of a large bundle exceeds its download cost. Budgeting by total page weight alone misses this, which is why the allocation here treats each asset type separately.
Budget for the connection your users actually have
Testing on a fast connection on a desktop machine produces a budget that fails everyone else. Median mobile connections are far slower than office wifi, and the fourth quartile of your audience is slower still. Setting the budget against a realistic connection is uncomfortable and it is the only version of the exercise that changes anything.
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Frequently Asked Questions
Why set a budget rather than just optimising?
Because a budget can be violated and a goal cannot. Performance work fails when nothing tells people they have broken it. A number in a build check does exactly that; an aspiration does not.
Why does JavaScript get a smaller budget than images?
Because it must be downloaded, parsed, compiled and executed on the main thread, while an image is decoded off it and can be lazy loaded. On a mid-range phone the parse and execute cost of a large bundle exceeds its download cost.
What connection speed should I budget for?
One your audience actually has, which is usually far slower than the office connection the site is built on. Budgeting against fast wifi produces a number that fails most real visitors.
Does the budget include third-party scripts?
It must. Analytics, tag managers, chat widgets and advertising frequently exceed the weight of the site itself, and they are the part of the budget nobody owns. A budget that excludes them measures the wrong thing.
What happens when a budget is exceeded?
Something has to come out, or the budget has to be renegotiated deliberately. The value is in forcing that conversation at the moment of the change rather than during a performance review months later.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Choose a target load time and connection speed to get a page weight budget by asset type.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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