How many extra tokens do function definitions add?

Each function definition adds 50-300 tokens to every API call (name + description + parameter schema). 10 tool definitions can add 1,000-3,000 tokens — 20-50% overhead on a 5K token call.

Do function call responses count as tokens?

Yes — both the function call JSON (output) and function result (input on next turn) are billed as tokens. A complex tool call cycle can add 500-2,000 tokens to your total cost.

How can I reduce function calling overhead?

Minimize tool descriptions, only include relevant tools for each query, use parallel tool calls to reduce round-trips, and cache tool schemas client-side to identify identical calls.

LLM Function Calling Token Overhead Calculator

Calculate the additional token cost introduced by LLM function/tool calling. JSON schema definitions, tool descriptions, and structured outputs add 200-2,000 tokens per call.

AI & SaaS Tools

SaaS Metrics

AI Compute

AI ROI

LTV:CAC Ratio

ARPU ($/month)

Monthly Churn %

CAC ($)

Gross Margin %

LTV

$3500

LTV:CAC

11.67

Payback

2.9 mo

Health

Healthy ✓

Benchmark: LTV:CAC ≥ 3 is healthy. Payback under 12 months is excellent. Under 1 means you lose money on each customer.

How to Use

Enter the values for the scenario you are assessing. Calculate the additional token cost introduced by LLM function/tool calling. JSON schema definitions, tool descriptions, and structured outputs add 200-2,000 tokens per call. Use the function calling overhead result to inform your calculation.

Related Calculators

GPT-4 Cost

AI & Tech

AI Agent Loop Cost

AI & Tech

Context Window Cost

AI & Tech

LLM Function Calling Token Overhead Calculator

LTV:CAC Ratio

How to Use

Related Calculators

Frequently Asked Questions

How many extra tokens do function definitions add?

Do function call responses count as tokens?

How can I reduce function calling overhead?