Infracost Cloud Cost Estimation

Show estimated monthly cloud costs in every Terraform pull request before merging. Catch expensive misconfigurations early, compare configuration options by cost, and enforce cost policies in CI.

Installation

# macOS
brew install infracost

# Linux / CI
curl -fsSL https://raw.githubusercontent.com/infracost/infracost/master/scripts/install.sh | sh

# Authenticate (get free API key at infracost.io)
infracost auth login

# Verify installation
infracost --version

Basic Terraform Integration

# Run a cost breakdown on your Terraform directory
infracost breakdown --path ./terraform

# Generate a plan file and use it (more accurate)
cd terraform
terraform init
terraform plan -out tfplan.binary
terraform show -json tfplan.binary > tfplan.json
infracost breakdown --path tfplan.json --format table

# Compare two branches
infracost diff --path . \
  --compare-to infracost-base.json \
  --format diff

Usage File for Variable Costs

# infracost-usage.yml — provide usage estimates for pay-per-use resources
version: 0.1
resource_usage:
  aws_lambda_function.api_handler:
    monthly_requests: 5000000          # 5M requests/month
    request_duration_ms: 150           # average 150ms execution
    memory_mb: 512
  aws_dynamodb_table.events:
    monthly_read_request_units: 1000000
    monthly_write_request_units: 100000
    storage_gb: 50
  aws_s3_bucket.assets:
    storage_gb: 500
    monthly_get_requests: 2000000
    monthly_put_requests: 50000

# Run with usage file
infracost breakdown --path . --usage-file infracost-usage.yml

GitHub Actions Workflow

# .github/workflows/infracost.yml
name: Infracost Cost Estimate

on:
  pull_request:
    paths:
      - 'terraform/**'

jobs:
  infracost:
    name: Run Infracost
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write

    steps:
      - name: Checkout base branch
        uses: actions/checkout@v4
        with:
          ref: '${{ github.event.pull_request.base.ref }}'

      - name: Setup Infracost
        uses: infracost/actions/setup@v3
        with:
          api-key: ${{ secrets.INFRACOST_API_KEY }}

      - name: Generate Infracost cost estimate for base branch
        run: |
          infracost breakdown \
            --path=./terraform \
            --format=json \
            --out-file=/tmp/infracost-base.json

      - name: Checkout PR branch
        uses: actions/checkout@v4

      - name: Generate Infracost cost estimate for PR branch
        run: |
          infracost breakdown \
            --path=./terraform \
            --format=json \
            --out-file=/tmp/infracost-pr.json

      - name: Post Infracost comment on PR
        uses: infracost/actions/comment@v1
        with:
          path: /tmp/infracost-pr.json
          compare-to: /tmp/infracost-base.json
          behavior: update  # update existing comment on re-run

Cost Policy — Block PRs Over Threshold

      # Add this step after the PR comment step
      - name: Check cost increase threshold
        run: |
          DIFF=$(infracost diff \
            --path /tmp/infracost-pr.json \
            --compare-to /tmp/infracost-base.json \
            --format json | jq '.diffTotalMonthlyCost | tonumber')
          echo "Monthly cost increase: $$DIFF"
          if (( $(echo "$DIFF > 500" | bc -l) )); then
            echo "ERROR: Cost increase ${DIFF}/month exceeds $500 threshold."
            echo "Please get finance approval before merging."
            exit 1
          fi

What Infracost Supports