'API' is one of those words developers say constantly and rarely explain. You'll hear that your booking system 'connects via an API', that a feature is blocked because 'their API doesn't support it', or that an integration will cost more because it 'needs custom API work'. Without knowing what the word means, you're negotiating in the dark.
The good news: the core idea is genuinely simple, and once it clicks you'll understand a huge amount about how modern websites actually work — and where the costs and limits come from.
The simplest possible explanation
An API is a defined way for two pieces of software to talk to each other. Think of a restaurant. You don't walk into the kitchen and cook; you give your order to a waiter, who takes it to the kitchen and brings back your meal. You don't need to know how the kitchen works — just how to order from the menu.
An API is that waiter and menu. Your website says 'charge this card £49.99' to the payment company's API, and gets back 'payment successful'. It never needs to know how the bank actually moves the money.
APIs you already rely on
You're using APIs all day without thinking about it. Here are everyday examples on business websites:
- Card payments — Stripe or PayPal's API processes the transaction so you never store card details.
- Maps — that embedded map showing your shop is Google's Maps API.
- Shipping rates — live courier prices at checkout come from the carrier's API.
- Email and CRM — a new enquiry flowing automatically into Mailchimp or HubSpot uses their API.
- Login with Google — 'Sign in with Google' buttons use Google's authentication API.
Why your website might need one
To avoid double data entry
If staff retype the same customer details into your website, your accounts package and your CRM, an API integration can pass that data automatically. Less typing, fewer errors, time saved.
To show live, accurate information
Real-time stock levels, live delivery prices, up-to-date availability — these stay accurate because an API fetches them on demand rather than relying on a number someone updated last Tuesday.
To add features you'd never build yourself
Payments, address lookup, fraud checks, mapping, SMS reminders — all available through APIs for a fraction of what building them from scratch would cost. This is a core part of modern web development: assembling reliable building blocks rather than reinventing each one.
To let other systems use your data
If partners, a mobile app or a customer portal need to read your data, you might expose your own API so they can access it in a controlled, secure way. This is common as businesses grow into a proper web application.
The two sides of API work
| Scenario | What it means | Typical cost |
|---|---|---|
| Consuming an API | Your site uses another company's API (e.g. Stripe) | Low; often just integration time |
| Integrating two systems | Connecting your site to a CRM or accounts tool | Moderate; depends on complexity |
| Building your own API | Letting apps/partners access your data | Higher; needs careful design and security |
What to watch out for
Most integration pain comes not from the API itself but from the gaps between systems — different fields, formats and rules that have to be reconciled.
A few practical cautions:
- API limits change. Providers can alter or restrict their APIs, occasionally breaking integrations. Reputable providers give notice; obscure ones may not.
- Security matters. Any API handling personal or payment data needs proper authentication and encryption. Don't let cost-cutting compromise this.
- Rate limits exist. Free tiers often cap how many requests you can make, which can bite at busy times.
Deciding what you need
You rarely need to 'build an API' — you usually need to connect existing systems sensibly. List the tools your business uses and where the same data gets entered twice or goes stale. Those friction points are exactly where an integration pays for itself.
When you're ready to scope it, share that list with a developer. Experienced web development specialists in our directory can tell you quickly whether an off-the-shelf connector exists or custom work is needed — and roughly what it'll cost — before you commit a budget.