Software

I would say "Help Desk Service levels" relate to all tiers and that means 5 Min response at first level (first call resolution hopefully), 2 hours response - 4 hour resolution - for tier 2, 4 hours response for tier 3. But when you say ratio for effective support i would say for every 50 non-IT employees 1 tech support person is sufficient. How you cover that spread depends on how ambidextrous your tier 2 and tier 3 are. I personally cover all tiers for 8 people in an off-site location from HQ. You could have a company with 100 people and 1 tier 3 person 1 tier 2 person and 1 tier 1 tech support - if they share responsibility for the help desk queue. However some higher tier people are not into taking tier 1 calls...

Thank you for your comment. 50:1 sounds reasonable for the support tiers. But actually the question was about "non-support" IT staff. For example what's about the personnel that is responsible for the new projects and stategy, IT analytics etc. or what's about the staff that is responsible for financial operations and external agreements in case of a large company? This staff is a part of the IT department but doesn't interract directly with end-users. How to determine their best ratio? Still use 50:1 or some other ratio?

sound like management functions nothing special wrt IT. (service strategy, financial management) Therefore I would suggest calculating a standard management footprint (darn few) for those based on resources.

Now why would you assume the other two wouldn't be handled by technical support staff? (security, new projects) My rule of thumb for staffing has always been by seat support as user counts (having multiple devices) would leave you short handed.

This rule of thumb just uses seats, but the underlying infrastructure is covered (network/server/vtc/peripheral/printing).

Start with 1 per 15 assuming no management infrastructure which includes -

1) Remote Control
2) Systems Imaging/centralized backup
3) Automated software/patch deployment
4) Centralized authentication with no admin privs for end-users
5) Helpdesk system with inventory control
6) Network management including centralized AV

Add 10 for each of the above that are present in your organization. E.G., if you have all of the above it's a 1 to 75 support ratio. This gives you leeway (assuming your staff has the requiste skills) for server management, helpdesk support, database management, small application development, end-user support, network management, security, e-mail administration, procurement/inventory control.

The above was developed in a geographically wide environment with centralized support staff. (average round trip time from base-point was 2.5 hours)

Charles Bundy, your comment is very helpful !
Thank you for sharing!

In my perspective in my public sector job, we've never even come close to 50:1 (or even 100:1). Budget constraints limit our ratio to about 300:1 on a good day... usually it's much worse.

I hope we can improve some day. I hear that it's good to have hope...

It depends on 1 - the level of support they need, 2 - the type of business and 3 - Is there anyone on permanent staff that knows enough about technology to take care of the little things without making a bigger mess?
I am the IT Support in a law firm of 38... I was supporting them on a consultant basis for 4 years, but rapid growth within the firm over the last 2 years dictated that they needed a full time IT person...
38 employees
4 locations
3 servers (application, exchange etc...)
48 PC's
13 Laptops
15 smart devices (Phones, tablets)
Voice/Data Infrastructure
Voice (Phone) support
Facility support... (They come to me when the air/heat isn't working the way that they want it too... LOL)
Vendor Management