# Coefficient of Utilization vs Finish Floor Height **URL:** \ **Category:** Project Planning & Design\ **Created:** [July 11, 2021, 8:28pm UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271 "2021-07-11T20:28:56Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Shikha](https://blackspectacles.com/uploads/images/13.png) [@Shikha](https://community.blackspectacles.com/u/Shikha)\ **Post date:** [July 11, 2021, 8:28pm UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/1 "2021-07-11T20:28:56Z")
Does the coefficient of utilization matter based on the mounting height of the fixture? I received a practice question, where it asked about the # of fixtures needed, and they gave me all the values, except the surface I was illuminating was 3 feet AFF. And the fixtures were mounted at 10 AFF and had a CU of 60% at that height. The question explicitly mentioned that the CU was at that height. My _guess_ is that since it has to do with the output of how much light is coming out of the fixture, the height won’t make the CU vary. Is this assumption correct? ---
**Author:** ![coachdarionziegler](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/coachdarionziegler/32/1272_2.png) [@coachdarionziegler](https://community.blackspectacles.com/u/coachdarionziegler)\ **Post date:** [July 13, 2021, 11:36am UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/2 "2021-07-13T11:36:57Z")
Hi @Shikha , Was this a Black Spectacles question? If so, do you know what form? A little more context might help me give you a more accurate answer! The coefficient of utilization is a measure of the efficiency of a luminaria to light a portion of the work plane, so mounting height of the fixture would definitely factor into the question. I just visited wikipedia to give myself a quick refresher on coefficients of utlization. > **[Coefficient of utilization](https://en.m.wikipedia.org/wiki/Coefficient_of_utilization)** > > A coefficient of utilization (CU) is a measure of the efficiency of a luminaire in transferring luminous energy to the working plane in a particular area. The CU is the ratio of luminous flux from a luminaire incident upon a work plane to that emitted by the lamps within the luminaire. As a ratio, the coefficient of utilization is unitless. > For example, some of the light emitted by a luminaire may exit away from the desired plane and is therefore wasted. A CU measures the light actually reachin... Thanks, Darion ---
**Author:** ![Shikha](https://blackspectacles.com/uploads/images/13.png) [@Shikha](https://community.blackspectacles.com/u/Shikha)\ **Post date:** [July 14, 2021, 2:55am UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/3 "2021-07-14T02:55:40Z")
It was actually in a third party material (Gang Chen’s PPD Mock Exam) question 56. Without writing the full question the key parts that mention CU are “The tenant wants 50 fc at desk level 3’-0” AFF…The CU is 60% when fixtures are installed at 10’-0" AFF…how many fixtures does the tenant need?" ---
**Author:** ![coachdarionziegler](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/coachdarionziegler/32/1272_2.png) [@coachdarionziegler](https://community.blackspectacles.com/u/coachdarionziegler)\ **Post date:** [July 14, 2021, 3:24pm UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/4 "2021-07-14T15:24:12Z")
Hi @Shikha , Ah ok… This is a bit outside my area of expertise. Maybe some of our PPD or PDD coaches can help? @coachadamcoers @coachmarkbailey @coachbryanhoward@coachrebeccasmith any assistance would be appreciated! Thanks, Darion ---
**Author:** ![coachbryanhoward](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/coachbryanhoward/32/942_2.png) [@coachbryanhoward](https://community.blackspectacles.com/u/coachbryanhoward)\ **Post date:** [July 18, 2021, 6:12pm UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/5 "2021-07-18T18:12:55Z")
Hi @Shikha ! This is a tricky one. There is a lot of information that goes into calculating the CU so it is difficult to believe that the question would be asking you to adjust the value based on heights. The CU is usually determined based on computer simulations and is variable based on surface area, distance from fixture to surface, and other inputs. Was there a formula provided to you as part of the question? Often times questions like these are really looking for you to find the correct formula that you need to find the answer, it could be possible that the CU information was put in there to throw you off. I suggest looking at the units you need for the answer, in this case # of fixtures, find the formula(s) that will provide that unit as an answer and determine if you have the inputs you need to run that formula. ---
**Author:** ![coachdarionziegler](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/coachdarionziegler/32/1272_2.png) [@coachdarionziegler](https://community.blackspectacles.com/u/coachdarionziegler)\ **Post date:** [July 19, 2021, 12:06pm UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/6 "2021-07-19T12:06:14Z")
Thanks @coachbryanhoward ! Great explanation. ---
**Author:** ![wildlandguy](https://blackspectacles.com/uploads/images/1.png) [@wildlandguy](https://community.blackspectacles.com/u/wildlandguy)\ **Post date:** [November 15, 2023, 5:06pm UTC](https://community.blackspectacles.com/t/coefficient-of-utilization-vs-finish-floor-height/3271/7 "2023-11-15T17:06:33Z")
i encountered a similar question recently. no formula was provided. had to use CU to find/convert to number of Foot Candles on the worksurface. how do you do that? thank you