How to Estimate Software Development Project Effort?

My Software Project Estimation Technique

Here is the software project estimation technique I successfully used on dozens of projects.

1. Define the main goal of you estimation. Is it planning, responding to RFP or preparation for next year budget? The answer defines how much risk you want to take, what to include into your estimate and how to present it.
• Be honest with yourself and your client. If any part of the project or even the project as a whole is impossible to estimate now due to lack of information, resources or other uncertainties, communicate it as early as possible.
• I you cannot estimate reliably, simply don't.
• Some clients may not want to see project buffers and may press on you to remove them to decrease their budget. You may want to hide project buffer by simply adding 5-15% to every task.
2. Define the the project scope being estimated. Define which works, pieces of functionality, target systems etc. are in and out of scope and under which assumptions.
3. Create a WBS. Noun-based one is preferred, it breaks the work down by pieces of functionality answering "What?" question, not "How?" or "When?". Simply speaking, this is a hierarchical list of features you are going to develop.
4. For every WBS line, ask developer(s) to estimate the time needed to develop the task. Prior to that agree what development must include (IDE setup, coding, bug-fixing, deployment, development test, code review etc.).
5. For every WBS line add time for Business Analysis, UX/UI design (30-100% of development time) and Test Activities (50-100% of development time). Refer to data from previous projects or expert opinion for exact percentage.
6. Add Risk Buffer for every WBS line (5-30% of BA + DEV + TEST, depending on how well-defined the task is or how many technical and other risks it bears).
7. Sum up the received time for every WBS line (QA + DEV + TEST + Risk Buffer).
8. Add time for Meetings (20-30% of development time), Project Management (10-20% of development time) and DevOps (5-10% of development time) for the whole project.
9. Add project buffer (5-30% of aggregated time, i.e. QA + DEV + TEST + Risk Buffer + Management + Meetings + DevOps) for the whole project. 
10. A rule of thumb is that your total time must be around three times higher than pure development time.
11.  Write down the scope you have estimated and assumptions you made on the way. This is an integral part of your estimate.

You may find an Excel template for this method at https://www.project-estimate.com/2022/12/software-project-estimate-template.html as well as tips, trick and other secrets of estimation process.