The cost of a contractor or consultant to do your job bears no resemblance to the cost of being in your job and having all the benefits of being a permanent employee.
Presumably you are smart enough to be able to work out how many hours you do a year (typically would be just under 2000 hrs a year), and therefore devise an hourly rate based on that. HOWEVER that is not the right way to work out your rate, but its probably what your HR person is going to compare it to. I would suggest as a quick guess you need to double your rate.
The right way to do it would be work out:
– How much you want to earn a year
– How much you want to put into a pension (sensible people suggest 10% of salary is the minimum)
– How much sickness type insurance might cost (I think this is dependent on age, health, duration, time before it kicks in, how long it lasts etc) – get a quote.
– Cost of insurance
– Cost of resources you will need to provide (e.g. laptops etc).
– Anything else you can think of.
Now divide that by the number of hours you expect to work each year, if you are looking at working for multiple people as a consultant assume that no more than 80% of your work time is actually earning and 20% is finding new business, issuing invoices etc. Short term projects / contracts need to charge more than long term reasonably secure work.
Now reverse the calculation for the minimum number of hours they are promising and see if it gives you enough income. Obviously they aren’t suggesting that you go it alone for your benefit – so it will be to let them cut hours / staff at short notice.
When I was in your shoes, I had some projects / people I liked working for who I would half my rate and work on some sort of deliverable / bonus / sweat equity / success fee basis with and some who I knew would try and screw everything possible out of me and more, or where the work was tedious or tortuous and I would charge them 50% extra.