SRS vs PRD: Which Requirements Document Do You Actually Need Before Hiring a Team?

Founders send me requirements in every shape: a voice note, a slide deck, a 40-page document copied from a template, a list of competitor apps. The question I hear most often is some version of "is this enough, or do I need a proper SRS?" The honest answer depends on who will read the document and what decision it has to support. Once you see what each document is for, the choice gets simple.

What a PRD is

A product requirements document (PRD) captures the product from the outside in. It names the problem, the users, the goals, the features for the first release and how you will know it worked. It is written for everyone: founders, designers, developers and investors should all be able to read it. I cover how to write one step by step in my guide to writing a PRD, and Atlassian has a useful overview of product requirements documents if you want a second view.

A good PRD usually includes:

• The problem and who has it
• User types and their main journeys
• Features for the first release, written as user stories
• What is out of scope for now
• Success metrics and launch goals

What an SRS is

A software requirements specification (SRS) describes the system from the inside. It turns each feature into precise, testable statements: inputs, outputs, rules, error cases, data fields, integrations and quality targets. The international standard for this work, ISO/IEC/IEEE 29148, describes what a software requirements specification should contain. It replaced the older IEEE 830 guidance that many SRS templates online still follow.

A typical SRS covers:

• Functional requirements for each feature, with rules and edge cases
• Data: what is stored, the fields, validation and retention
• External interfaces: payment gateways, APIs, hardware, other systems
• Non-functional requirements such as speed, scale, security and availability
• Constraints: platforms, regulations, hosting region, technology limits

SRS vs PRD side by side

Here is the same requirement at both levels. In a PRD: "As a customer, I can reschedule my booking up to a day before the slot." In an SRS: "The system shall allow a customer to change the slot of a confirmed booking until 24 hours before its start time. After that, the reschedule option is hidden and the customer sees a message to contact support. The original slot is released immediately. A confirmation is sent by email and push notification."

Which one do you need?

This is the rule of thumb I use with founders:

• Startup or new product, first version: write a PRD. Add your non-functional targets. Let the development team produce the detailed specification during discovery, because they will find the questions you have not thought of.
• Fixed-price contract: you need SRS-level detail before signing, because a fixed price is only fair when both sides agree exactly what is included. Our guide on fixed price vs time and materials explains why.
• Regulated work such as healthcare, finance or government: an SRS is often expected, because auditors need traceable, testable requirements.
• Replacing an existing system or ERP: an SRS helps, because the old system already has hundreds of rules that must be written down before anyone can rebuild them.
• Formal tender: you may send an RFP with a PRD attached, and ask bidders how they will produce the SRS.

What to send a development company for a quote

You do not need a 40-page SRS to get a useful estimate. You need enough for two companies to picture the same product. This checklist covers it:

1. A one-paragraph summary of the problem and the users
2. The main user journeys, written as user stories
3. What is in the first release and what is not
4. Platforms: iOS, Android, web, admin panel
5. Integrations: payments, maps, messaging, existing systems
6. Rough scale, security and privacy needs
7. Any deadline and the reason for it

With that in hand, the estimates you receive will be comparable. To see what moves a budget before you send anything, use our project cost guide or the app cost calculator.

Common mistakes

• Copying an SRS template and filling every heading. You end up with long sections nobody reads and the important rules missing.
• Writing technical choices into the PRD. "Use microservices" is a decision for the team, not a requirement. Write the need, such as the expected load, instead.
• Skipping the "why". An SRS without business goals lets the team build exactly what was written and still miss the point.
• Treating either document as frozen. Requirements change as you learn. Keep one source of truth and record each change, or you invite scope creep.
• No acceptance criteria. If a requirement cannot be tested, nobody can say when it is done.

How we handle it at Creuto

At Creuto, we start from whatever you have, even a voice note. In the first call we go through it with you, and you get a clear written proposal within 24 hours. During discovery we turn the PRD into the detailed, testable requirements the build needs, so you do not have to learn SRS writing to get the product right. The first working version usually follows in about 4 weeks, your code lives in your own accounts from day one, and a 60-day warranty covers the product after launch.

If you are about to write requirements, start with the PRD. It is the document only you can write. The technical detail is our job.