If you’re building AI apps in 2026, you’ve probably experienced one of two things:
You got your OpenAI or Anthropic API bill and nearly had a heart attack because you miscalculated the output token ratio.
You wanted to estimate a massive, proprietary system prompt, but hesitated to paste it into random-token-counter-xyz.com because you have no idea where that text is actually going.
I got tired of both.
System prompts are essentially the source code and business logic of modern AI apps. Pasting them into a cloud-based text area without an API key just to count words is a massive security vulnerability.
So, I built a 100% client-side alternative: The Zero-Trust AI Token & Cost Calculator.
The Problem with Most Token Counters
Most tools out there either require you to paste your actual API key (hard pass), or they send your text payload to a backend server to run it through an official tokenizer library like tiktoken.
But for simple cost estimation and budgeting, making a network request is overkill and a privacy nightmare.
The Solution: A Local Heuristic Algorithm
I wanted something that runs instantly in the browser memory and gives me a 95% accurate cost projection for GPT-4o, Claude 3.5 (Opus/Sonnet), and Gemini.
Instead of bundling a heavy WASM tokenizer, I wrote a lightweight heuristic engine in vanilla JavaScript. It works entirely offline:
English/Latin text: It splits by whitespace and applies the standard industry heuristic: 1 word ≈ 1.33 tokens. It also separately counts punctuation marks, as LLMs usually tokenize them individually.
CJK (Chinese, Japanese, Korean) Support: CJK characters are notorious for eating up tokens. The script uses Regex (/[\u4e00-\u9fa5...]/g) to isolate them and applies a conservative 1.5 tokens per character multiplier based on modern 2026 tokenizer behaviors.
It’s not byte-for-byte perfect to OpenAI's exact tokenizer, but for financial modeling and prompt-trimming, it’s completely frictionless and instantly accurate enough.
Cost Projection is More Than Just Input
The biggest mistake junior devs make is looking at the "$5 per 1M tokens" sticker price and ignoring the output. Generating text is computationally expensive, and API providers charge 3x to 5x more for output tokens. (Looking at you, Claude 3.5 Opus, with your $75/1M output rate).
The tool allows you to:
Paste your giant prompt to get the Input Cost.
Set your expected Output Tokens length.
Select the model (GPT-4o, Claude, Gemini).
Instantly see the exact USD cost for a single run, AND a bulk projection for 1,000 API calls to help you price your SaaS tiers.
Try it out (Safely)
You can test it out here: Mini-Tools.uk AI Token Calculator
Turn off your Wi-Fi, paste your most highly classified, NDA-protected system prompt into the box, and watch the pricing calculate instantly. Your data never leaves your .
Let me know what you guys think, or if there are any obscure open-source models whose pricing tiers I should add to the dropdown!