Billing used to be simple: count the seats, send the invoice, done. But let's be honest, that doesn't work anymore.

The moment you introduce usage complexities like API calls, GPU minutes, storage tiers, workflows, requests per second, or feature-level entitlements, traditional billing tools just give up.

They weren't built for this. Your systems can't meter at a granular level, can't enforce prepaid credit limits, and definitely can't handle hybrid pricing models like subscription + credits + usage all working together.

So what happens? Engineering teams end up managing the billing instead of building the product. There's no single source of truth, just a mess of data scattered everywhere.

And closed platforms? They're not doing you any favors. Vendor lock-in and lack of pricing control trap you in rigid pricing models. You can't adjust, you can't experiment, you can't move fast. You request a feature and wait... and wait... and wait.

Even if you want out, the switching costs are brutal. Migration is a nightmare. So you stay stuck, innovation slows down, and operational flexibility becomes a distant dream.

There has to be a better way.

In this blog we will talk about all the open source alternatives you have for usage based billing.

Note: Even though this post is published on Flexprice, it’s not a biased roundup. We’ve evaluated every tool on its technical merit, flexibility, and developer experience exactly how we’d expect anyone building serious AI infrastructure to do.

TL;DR

• Billing has shifted beyond seat-based pricing; AI workloads demand granular metering for API calls, GPU seconds, tokens, storage, and more.

• Closed, proprietary billing tools break with usage complexity due to limited programmability, vendor lock-in, and slow feature delivery.

• Open-source billing is rising because it offers transparency, control, customization, and lower switching cost.

• Flexprice: Developer-first, open source, AI-native real time billing platform that handles real-time metering, credits, hybrid pricing, and programmable billing logic end-to-end and is equipped with all enterprise grade features. You can integrate Flexprice with your product in days.

• OpenMeter: High-volume, real-time usage metering and quota enforcement engine designed to plug into your existing billing stack.

• Kill Bill: Enterprise-grade, deeply customizable billing and payment orchestration system with a powerful plugin ecosystem.

• UniBee: Lightweight, easy-to-deploy subscription + usage billing tool ideal for startups with simple billing needs.

• jBilling: Modular, enterprise-focused billing engine built for complex workflows and ERP/CRM integrations across high-volume industries.

• A modern usage-based system requires: metering, aggregation/rating, pricing + entitlements, invoicing, payment sync, and CRM/finance integration.

• Flexprice stands out for AI-native products with real-time metering, credit wallets, hybrid pricing models, contract overrides, audit-safe ledgers, and self-hosting.

• Open-source billing is growing faster as SaaS and AI companies demand flexibility, transparency, and faster pricing iteration without vendor lock-in.

What is Usage-Based Billing?

Usage-based billing is a pricing strategy, where you charge your customers based on their consumption and not a flat or seat based fee.

The shift is happening because AI is changing how software companies charge customers. A Business Insider article talks about how AI compute cost spikes are making flat seats unrealistic.

Businesses always look at aligning with customers’ needs and aspirations; usage-based billing has become the most preferred approach as customers value transparency, fairness and accurate billing.

Usage-based billing offers a dynamic way to connect pricing with value delivered, helping businesses attract a broader customer base.

Key Components of a Usage-Based Billing System

• Metering & Data Ingestion: Accurately tracking and collecting customer usage data in real time or batches. It involves measuring metrics like API calls, storage and GPU seconds consumed.

• Aggregation & Rating: Usage data is aggregated for a billing period and processed through rating rules to calculate charges. This supports complex pricing such as tiered rates and volume discounts.

• Pricing & Entitlements: Pricing models define how usage translates into charges, including hybrid approaches that combine subscriptions with usage fees and credits. Entitlements determine which features or usage levels apply to each customer.

• Invoicing & Payments: : After rating, invoices are generated reflecting detailed usage and charges. This includes billing cycles, tax calculations, and integration with payment gateways for seamless collections.

• Integration with Finance and CRM: The billing system must connect to finance and CRM software to sync customer records, revenue recognition, payment status, and other financial workflows for end-to-end management.

Top 5 Open-Source Usage-Based Billing Tools

1. Flexprice

2. OpenMeter

3. KillBill

4. UniBee

5. jBilling

1. Flexprice

Flexprice is a developer-first, open-source billing platform designed for AI, API, SaaS and usage-heavy products. It handles usage-based, credit-based, outcome based and hybrid pricing models in one place.

At its core, Flexprice is an usage based solution billing engine designed for AI-native, SaaS and infrastructure- intensive companies. It is capable of handling high-volume event ingestion in real-time with accuracy. Flexprice is designed for companies that need programmable billing logic, consent-based entitlements, and granular overrides.

It supports accurate usage metering such as API calls, inference minutes, or GPU time, and connect it seamlessly to invoicing.

If you need granular control over how you price and bill AI and SaaS products. Unlike black-box billing tools such as Stripe and Chargebee, it’s open source, giving you the freedom to implement and test custom pricing models without vendor lock-in.

Flexprice is enterprise-ready from day one, offering built-in audit logs, role-based access control, and automated revenue workflows that scale seamlessly from startups to large enterprises. AI Teams appreciate the fully programmable, API-first architecture covering everything from metering to billing workflows.

For AI teams that require transparent, flexible, and developer-friendly pricing, Flexprice provides the complete stack to help you monetize confidently while having full control over your billing logic.

One of the key differentiators that Flexprice has is that it literally takes you 2-3 days to integrate with your current system so you really don’t have to wait for your next pricing or billing update.

Key Features

• Real-Time Event Ingestion: Capture usage events instantly with rich metadata for accurate, up-to-date billing and analytics.

• Enterprise grade features: Enterprise-grade control, with the observability, security, and scalability your billing stack needs to handle billions of events without breaking a sweat.

• Credit Wallet Management: Manage prepaid credits flexibly with expiration, credit rollover, and prioritized deductions for customer convenience.

• Metered Features with Multi-Metric Pricing: Support detailed usage tracking across multiple metrics like API calls, GPU time, and token counts with flexible pricing models.

• Contract-Level Overrides: Customize pricing and billing terms per contract, enabling tailored commitments and overrides for individual customers.

• Built-in Invoice Generation: Automatically create clear, compliant invoices linked directly to metered usage, subscriptions, and credits.

• Customer Dashboards API: Provide real-time visibility into consumption and charges through dashboards for both customers and internal teams.

• Built for AI and Agentic Teams: Specialized support for AI-native teams including inference tokens, GPU usage, and agentic workload metering.

• SOC2: Flexprice follows SOC 2-aligned security practices for handling billing and usage data, helping you meet enterprise compliance expectations.

• Payment Gateway integration: Flexprice integrates with payment processors like Stripe, Razorpay and many others so you can connect metered billing and invoicing to your existing payment flows.

• High volume event handling: Flexprice’s metering pipeline is designed for large-scale AI and API workloads, ingesting and aggregating high-volume usage events of over 20bn+ every month in real time for accurate billing.

• Ramped Contracts:Flexprice lets you configure ramped contracts with changing prices or minimums over time so enterprise deals can move from pilot to scaled phases without custom code

• Parent-Child Accounts: Flexprice supports account hierarchies, allowing multiple child workspaces or teams to roll up into a single parent billing profile with shared credits and usage.

• Granular usage filtering: Flexprice applies different prices from a single event stream using metadata like model, region, or usage type, so you can bill precisely for each variant without separate schemas.

• Faster pricing iteration: Flexprice’s open and configurable pricing engine lets you ship or adjust plans and experiments in less than 30 minutes whereas other platforms take days for the same

• Flexprice combines hands-on technical support with an active community so your team can get implementation help and billing guidance across time zones.

• Feature entitlements: Flexprice lets you define and enforce feature flags and usage-based entitlements so you can gate access, enforce limits, and manage trials from a single system.

• Multi currency support: Flexprice helps you localize pricing and charge different rates across regions and currencies for global customers.