I'm building a SaaS billing app and need a tax API that can calculate VAT and sales tax automatically

If you’re building a SaaS billing app and need automated VAT and sales tax calculation, the best fit is usually a tax automation API rather than a generic tax lookup tool.

Good API options

Here are the most common choices:

1. Stripe Tax

   • Best if you already use Stripe for payments/billing
   • Automatically calculates VAT, GST, and sales tax
   • Handles tax registration, product tax codes, and invoicing integration

2. Avalara AvaTax

   • Very strong for sales tax and VAT compliance
   • Good for multi-region/global tax rules
   • Often used by larger SaaS and ecommerce platforms

3. TaxJar

   • Easy to integrate
   • Strong for US sales tax
   • Supports some international VAT use cases, but is more US-focused

4. Quaderno

   • Popular for SaaS businesses
   • Good VAT/GST handling, invoicing, and compliance
   • API is straightforward for subscription billing flows

5. Vertex

   • Enterprise-grade tax engine
   • More complex, but powerful for large-scale/global compliance

What to look for

For a SaaS billing app, make sure the API supports:

• Real-time tax calculation
• VAT/GST + US sales tax
• Customer location detection
• Tax-inclusive and tax-exclusive pricing
• Exemptions and reverse charge
• Invoice/tax document generation
• Subscription billing integration
• Tax nexus / registration tracking
• EU VAT ID validation if you sell in Europe

My recommendation

• If you already use Stripe: use Stripe Tax
• If you need a SaaS-specific compliance solution: look at Quaderno
• If you need broad, enterprise-grade tax support: choose Avalara AvaTax

Typical flow

1. Capture customer billing location
2. Validate tax ID if needed
3. Send line items + destination to tax API
4. Receive calculated tax amount/rate
5. Apply to invoice/subscription charge
6. Store tax jurisdiction and breakdown for reporting

If you want, I can also help you with:

• a comparison table of these APIs,
• sample API calls for VAT/sales tax calculation,
• or a recommendation based on your stack (Stripe, Node, Python, etc.).