Best Open Source Billing and Invoicing Tools in 2026

TL;DR

Stripe Billing charges 0.5–0.8% of recurring revenue — at $100K MRR, you're paying $500–800/month just for the billing layer on top of Stripe's standard processing fees. Lago is the best open source alternative for SaaS subscription and usage-based billing. InvoiceNinja wins for freelancers and service businesses needing professional invoicing. Kill Bill handles enterprise-grade subscription complexity.

Key Takeaways

Lago (AGPL-3.0, 7K+ stars) specializes in usage-based and subscription billing with real-time metering — the open source Chargebee/Recurly
Kill Bill (Apache-2.0, 4.5K+ stars) is enterprise-grade subscription management with revenue recognition, dunning, and a mature plugin ecosystem
InvoiceNinja (AAL, 8K+ stars) is a complete invoicing platform with quotes, proposals, expense tracking, time tracking, and a client portal
Crater (AGPL-3.0, 7.7K+ stars) is the simplest self-hosted invoicing tool for small businesses replacing Wave or basic FreshBooks functionality
Stripe Billing fees at $100K MRR: $6,000/year (0.5%); self-hosted Lago: $72/year (VPS) — $5,928 annual savings
None of these tools replace your payment gateway — they sit on top of Stripe/PayPal to handle billing logic

Understanding the Billing Stack

It's important to distinguish between:

Payment gateway — Processes the actual card transaction (Stripe, PayPal, Braintree). Always required.
Billing/subscription management — Manages plans, trials, upgrades, downgrades, proration, invoices, and dunning (Stripe Billing, Chargebee, Recurly).
Invoicing — Creates and manages professional invoices, quotes, and payment tracking (FreshBooks, QuickBooks).

The tools in this comparison replace category 2 and 3 — not the payment gateway. You still need Stripe or PayPal to process payments. What you eliminate is the layer sitting on top that charges you a percentage of recurring revenue for the privilege.

Lago — Best for SaaS Billing

Lago is the open source answer to Chargebee and Recurly. It handles subscription management, usage-based billing, and invoicing for SaaS companies. The differentiator is real-time metering: Lago can process millions of usage events per hour and bill customers based on exactly what they consumed.

Usage-based billing is the pricing model many modern SaaS companies want but struggle to implement. API calls, compute seconds, storage bytes, active users, messages sent — any metered dimension can be tracked in Lago and priced in flexible ways:

Per-unit (flat rate per API call)
Volume tiers (cheaper per-unit as consumption grows)
Graduated (different rates per tier of consumption)
Package-based (bundles of units at a flat price)
Prepaid wallets (customers pre-purchase credits)

# Lago Docker Compose
services:
  lago:
    image: getlago/api:latest
    environment:
      - DATABASE_URL=postgresql://lago:password@postgres:5432/lago
      - REDIS_URL=redis://redis:6379/0
      - SECRET_KEY_BASE=your-secret-key-base
      - LAGO_ENCRYPTION_PRIMARY_KEY=your-encryption-key
      - LAGO_API_URL=https://lago.yourdomain.com
      - LAGO_FRONT_URL=https://app.lago.yourdomain.com
    ports:
      - "3000:3000"
  lago-front:
    image: getlago/front:latest
    ports:
      - "80:80"
  postgres:
    image: postgres:15
    volumes:
      - lago_db:/var/lib/postgresql/data
  redis:
    image: redis:7-alpine
volumes:
  lago_db:

Lago's metering API accepts events that you send from your application:

# Send a usage event to Lago
curl -X POST https://lago.yourdomain.com/api/v1/events \
  -H "Authorization: Bearer your-api-key" \
  -H "Content-Type: application/json" \
  -d '{
    "event": {
      "transaction_id": "tx_12345",
      "external_customer_id": "customer_abc",
      "code": "api_call",
      "timestamp": 1709856000,
      "properties": {
        "endpoint": "/v1/completions",
        "tokens": 1500
      }
    }
  }'

Lago aggregates these events, applies billing rules, and generates invoices at the end of each billing period. The invoice PDF is generated automatically and sent to customers.

Key features:

Subscription plan management
Usage metering (real-time event ingestion)
Flexible pricing models (per-unit, graduated, volume, package, prepaid)
Coupon and credit system
Prepaid wallet credits
Invoice generation and delivery
Dunning and retry rules
Webhook events for billing lifecycle
REST API
Stripe/Adyen/Braintree payment gateway integrations

Kill Bill — Enterprise Subscription Management

Kill Bill has been the open source enterprise billing standard since 2012. The platform is battle-tested: it's been deployed by Groupon, Zendesk, and hundreds of other companies for complex subscription management.

The plugin architecture is Kill Bill's defining feature. Every business has billing logic that doesn't fit a standard model — custom dunning sequences, specialized tax calculations, industry-specific pricing structures. Kill Bill's OSGi plugin system lets developers add this logic as modular, independently deployable plugins.

Revenue recognition is a Kill Bill capability that simpler tools lack. For companies that need to comply with ASC 606 or IFRS 15 (the accounting standards for recognizing subscription revenue over time), Kill Bill tracks deferred revenue and generates the reports accountants need.

Multi-currency billing handles the complexity of charging customers in their local currency while reporting revenue in your functional currency. Exchange rate management, currency rounding rules, and multi-currency invoice formatting are all built in.

Kill Bill is Java/Maven-based and requires more operational expertise than Lago. The setup involves a Java application server, MySQL/PostgreSQL, and plugin installation via the Kill Bill admin UI or API.

Key features:

Subscription lifecycle management (create, upgrade, downgrade, cancel)
Proration for mid-cycle plan changes
Multi-currency billing
Revenue recognition (ASC 606)
OSGi plugin architecture
Dunning and retry logic
Tax integration (Avatax, TaxJar via plugin)
Custom billing periods
One-time and recurring charges
REST API

InvoiceNinja — Best Professional Invoicing

InvoiceNinja is the most complete open source invoicing platform, with features that rival FreshBooks and QuickBooks invoicing:

Full invoice workflow: Create quotes/proposals → convert to invoices → track payment → generate receipts. Each step has professional PDF templates, client communication history, and payment link generation.

Client portal gives customers a branded web interface to view all their invoices, download PDFs, and pay online. This eliminates the manual "send PDF, wait for check" cycle that wastes hours for service businesses.

Time tracking and expenses integrate directly into invoice creation. Log hours on a project, mark expenses as billable, and turn both into line items on the next invoice automatically. This makes InvoiceNinja the right tool for agencies, consultants, and any business that bills by time.

# InvoiceNinja Docker
services:
  server:
    image: invoiceninja/invoiceninja:latest
    environment:
      - APP_URL=https://invoicing.yourdomain.com
      - APP_KEY=base64:your-app-key
      - DB_HOST=mysql
      - DB_DATABASE=ninja
      - DB_USERNAME=ninja
      - DB_PASSWORD=password
      - MAIL_MAILER=smtp
      - MAIL_HOST=smtp.yourprovider.com
    volumes:
      - ninja_public:/var/www/app/public
      - ninja_storage:/var/www/app/storage
    ports:
      - "80:80"
  mysql:
    image: mysql:8
    environment:
      MYSQL_DATABASE: ninja
      MYSQL_USER: ninja
      MYSQL_PASSWORD: password
      MYSQL_ROOT_PASSWORD: rootpassword
    volumes:
      - ninja_db:/var/lib/mysql
volumes:
  ninja_public:
  ninja_storage:
  ninja_db:

Payment gateway integrations include Stripe, PayPal, Authorize.net, Braintree, WePay, and 15+ others. Accept ACH bank transfer payments alongside card payments.

Key features:

Quote → invoice → receipt workflow
Client portal (branded)
Time tracking with billable hours to invoice
Expense tracking with billable expenses
Recurring invoices (auto-send on schedule)
Payment gateway integrations (15+)
Multiple currencies
Tax rate management
PDF customization with Blade templates
Mobile apps (iOS and Android)
REST API

Crater — Best Small Business Invoicing

Crater is the simplest tool in this comparison — a clean Laravel application for creating and tracking invoices, managing customers, and recording payments. The UI is modern and intuitive, making it accessible for business owners who aren't accountants.

The feature set covers exactly what small businesses need: invoices with line items and taxes, expense tracking, payment recording, and basic reporting on outstanding vs. paid amounts. What it intentionally excludes is the complexity of Kill Bill or the developer-API focus of Lago.

Full Comparison

Feature	Lago	Kill Bill	InvoiceNinja	Crater
License	AGPL-3.0	Apache-2.0	AAL	AGPL-3.0
Min RAM	1 GB	2 GB	512 MB	256 MB
Usage Billing	✅ Native	✅ Plugin	❌	❌
Subscriptions	✅	✅ Enterprise	✅ Recurring	✅ Recurring
Revenue Recognition	❌	✅	❌	❌
Client Portal	❌	❌	✅	✅ Limited
Time Tracking	❌	❌	✅	❌
Multi-currency	✅	✅	✅	✅
Dunning	✅	✅	✅	❌
REST API	✅	✅	✅	✅
Payment Gateways	Stripe, Adyen	15+ plugins	15+	Stripe, PayPal

Decision Framework

Choose Lago if: You're building a SaaS product with subscription or usage-based billing. The metering API and flexible pricing models are the features Chargebee and Recurly charge 0.5–1% for.

Choose Kill Bill if: You have complex enterprise billing requirements — revenue recognition, multi-currency, sophisticated dunning, or custom billing logic via plugins.

Choose InvoiceNinja if: You're a freelancer, agency, or service business that needs professional invoicing with a client portal, time tracking, and expense management.

Choose Crater if: You want the simplest self-hosted invoicing with a clean UI. Replacing Wave or basic FreshBooks.

Cost Savings

MRR	Stripe Billing (0.5%)	Lago Self-Hosted	Annual Savings
$10K	$50/month	$8/month	$504
$100K	$500/month	$8/month	$5,904
$500K	$2,500/month	$15/month	$29,820

At $500K MRR, Lago self-hosted saves nearly $30,000/year compared to Stripe Billing's 0.5% fee.

Best Open Source Billing & Invoicing Tools 2026

Best Open Source Billing and Invoicing Tools in 2026

TL;DR

Key Takeaways

Understanding the Billing Stack

Lago — Best for SaaS Billing

Kill Bill — Enterprise Subscription Management

InvoiceNinja — Best Professional Invoicing

Crater — Best Small Business Invoicing

Full Comparison

Decision Framework

Cost Savings

The SaaS-to-Self-Hosted Migration Guide (Free PDF)