A Lambda function ARN carries more moving parts than most people remember on the first try: a region, a 12-digit account id, the literal word function, the function name, and — optionally — a version number or alias tacked on with one more colon. Lambda ARN Generator takes those pieces as separate form fields and assembles them into the exact ARN Lambda expects, so you don’t have to hand-count colons or remember whether the version goes before or after the function name (it goes after, with its own colon).
The form has four fields: region, account id, function name, and an optional version-or-alias suffix. Leave the suffix blank and you get the unqualified ARN — arn:aws:lambda:region:account:function:name — which always resolves to whatever is currently deployed as $LATEST. That form is what most people reach for when writing an IAM policy that should apply to a function regardless of which version is live, or when wiring up a Lambda trigger during initial development. Fill in the suffix with either a published numeric version (7) or an alias name (PROD, live, staging) and the tool appends it as a third colon-separated segment — arn:aws:lambda:region:account:function:name:PROD — pinning the ARN to a specific, known version. This qualified form matters anywhere you want guaranteed behavior: an API Gateway Lambda proxy integration pointed at a stable alias, an event source mapping that should never silently pick up an untested deploy, or a resource-based policy statement scoping lambda:InvokeFunction to exactly one alias.
Unlike S3, Lambda ARNs are fully regional and account-scoped — the same function name can exist independently in us-east-1 and eu-west-1, or under two different AWS accounts, so both the region and account id are required fields here rather than optional extras. The region field expects a real AWS region code shape (like us-east-1); the account field expects exactly 12 digits. Both are checked at build time by the same validator used across every ARN tool in this family, so a region that doesn’t look like a real region code or an account id with the wrong digit count gets flagged before you copy the ARN into a policy or template where the mistake would be harder to spot.
The tool sticks to the one shape most people actually need — function:name[:version-or-alias] — rather than trying to cover every Lambda-adjacent ARN type. Layer ARNs use a different resource type entirely (layer:name:version) and aren’t built here; if you need one, the underlying pattern is similar but distinct enough that conflating the two in one form would be more confusing than helpful.
Once the required fields are filled, a live preview shows the ARN updating as you type, before you even click Build. Clicking Build runs the result through structural validation, surfaces any warnings about the region or account format, and reveals a one-click Copy button. A Load sample button fills in a realistic region, account, function name, and alias so you can see the qualified form immediately, and Clear resets everything for the next function. As with every tool in this family, construction and validation happen entirely in your browser — the account id you type, which uniquely identifies an AWS account, is never transmitted anywhere.