Developer Tools
JSON Size Calculator
JSON Size — JSON Size Calculator measures the byte size of the JSON you paste, with a gzipped-size estimate. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Use JSON Size Calculator to size a data payload or transfer: give it your JSON text, press Calculate, and read off the size and an estimated transfer time — all worked out on your device.
Browser-side developer and utility helpers for everyday and learning use. Everything runs locally in your browser and nothing you enter is uploaded. Validate production or security-critical output with trusted, official tools before relying on it.
About this tool
JSON Size Calculator measures the byte size of the JSON you paste, with a gzipped-size estimate. That is the whole of the calculation — the mistakes happen in the inputs. It matters when a format, encoding or size has to be exactly right for the system reading it. Worked example: A JSON string of 2,048 characters is about 2,048 bytes (2 KB) in UTF-8 for ASCII content; multi-byte characters add to that. Use the calculator with your own figures; it runs entirely on your device.
How to use
- Paste a JSON object, array, or value.
- Let the tool parse and validate it.
- Run it to measure the encoded sizes.
- Compare the compact and pretty byte sizes.
Why use the JSON Size Calculator
Instant
Each result is computed on your device the moment you run the tool.
Private
The text and numbers you enter stay in your browser and are never uploaded.
Free
No signup and no paywall to use any tool in this set.
Works offline
Once the page has loaded it keeps working on a weak or dropped connection.
Built for developers
Plain inputs and outputs you can copy straight into code, configs, or notes.
Honest by design
Conversions and heuristics are for everyday and learning use; verify security-critical output with trusted, official tools.
Common uses
Use it for trimming API response weight
Use it when debugging a large response
Use it when estimating transfer time
Use it before shipping a new endpoint
JSON Size in storage-cost planning
Use it for front-end performance work
Technical notes
JSON Size Calculator measures the byte size of the JSON you paste, with a gzipped-size estimate.
Results are reference values; validate against your own environment before relying on them in production.
Because it runs locally, results appear instantly with no server round-trip.
Worked example
A JSON string of 2,048 characters is about 2,048 bytes (2 KB) in UTF-8 for ASCII content; multi-byte characters add to that.
FAQ
Does it measure bytes or characters?
It reports UTF-8 bytes, which is what storage and transfer limits use, and also shows the character count for reference.
What if my JSON is invalid?
The tool reports that the input is not valid JSON, along with the parser message, instead of guessing a size.
Why are compact and pretty sizes different?
Pretty-printing adds spaces and new lines for readability, so it takes more bytes than the minified form.
Is my JSON sent to a server?
No. Parsing and measuring happen entirely in your browser and nothing is uploaded.