How to calculate the project timeline if the project is large and non-standard?

Question

Project management

We develop non-standard web projects for clients. Basically automating some part of their business processes. Due to the fact that the projects are non-standard, the question arises about the timing - what to say?
Once again it happens that we say - and we do not meet the deadlines. It turns out that the module requires more hours to develop than expected. The client also plans his activities based on our deadlines.
As a result, we have not the highest quality code, long deadlines that knock down the deadlines for other projects (which hits the pocket), hassle, sometimes departed customers (which also hits the pocket).
What can be done?
Thanks in advance.

Answer 1

1) do detailed TK.
2) divide the TK and distribute it to those who will implement it.
3) each link that makes the project to provide time to get acquainted with the project and ask to determine the approximate time for which he will be able to implement his part.
4) collect time data from employees - analyze.
and only now can you provide an estimated development time to the client.
ps the essence of this method is that employees are immediately familiar with what they have to do. and in 90% of cases there are no situations as you described "It turns out that the module takes more hours to develop than expected."
pps It's clear that this method will also not give you a 100% accurate deadline, but many problems will definitely disappear.

Answer 2

Read Brooks "The Mythical Man-Month"
About half a century ago, we already knew that estimates in IT were very optimistic.
Method 1:
Multiply your grades by 3. This is what I do.
Method 2:
Divide the work into small pieces. Evaluate (and pay piecewise).

Answer 3

Calculate in the head approximately with a margin, but warn of the possibility of unaccounted for complexity. After that, the deadlines will, as it were, have force majeure in cases of not laying on time. And you can always say that you warned about - the possibility of unaccounted for complexity.

Answer 4

No way.

Answer 5

1. Learn from your mistakes and successes. There is no evaluation algorithm, there are methods, but they require evaluation experience. Without experience of an estimation all the same there will be an incorrect result.
2. Read "Waltzing with the Bears". There is a methodology for calculating Riskology, from which it will at least be clear how it works in general.

Answer 6

If the project is more than half a year, then it will be much more effective to guess by listening to a 15-minute presentation than to engage in assessments and attempts to make a plan out of it (Not a joke.) If our company sees a project that needs to be analyzed for more than a week, it immediately refuses to give an assessment and says that it is possible develop for a year or two, etc. And there are clients who agree and work. Mostly experienced. Those who are not experienced run away to where they beckon with a fixed price and specific numbers.