Rack sizes – how to calculate how many U you need?

Hosting your IT in a data centre using colocation services is an opportunity to upgrade resiliency and reliability, while saving you money. However, to make this saving you need to make sure you’re taking on the right size rack for your equipment and not paying for more than you need.

How are racks measured?

If you’re here to find out what size rack of lamb you need for four people, then you’re probably very confused and it’s best you click away. But if you’ve got some IT equipment and don’t know where to begin with calculating the rack size you need, then this is the article for you.

The racks in a data centre are measured in U (short for Unit). One U is 1.75 inches tall, and servers (as well as other IT equipment) are often built in 1.75-inch increments to easily translate into U.

Racks come in different sizes when it comes to commissioning colocation services. Rack sizes vary between data centres, but they are typically between 42U and 47U tall, and are divided into full, half, and quarter racks:

  • Full Rack – Between 42U and 47U
  • Half Rack – Typically 22U
  • Quarter Rack – Typically 11U

If you only need a handful of U but want to benefit from the resilient infrastructure of a data centre, some colocation providers will offer shared colocation services. This allows you to take a few U in a larger rack that you share with other businesses.

How to calculate what rack size you need

The rack size you need depends on how many U of space your equipment takes up. The start of this calculation is very simple since each server you have should be 1U tall, so you can start off by counting the number of servers you have and noting that down.

The other equipment that supports your IT system might be slightly more complicated to calculate, this can include:

  • Hardware-based firewalls
  • Other cyber security equipment
  • Switches and routers
  • Patch panels
  • Power bars

Each of these pieces of equipment will take up space. Especially if any of them are taller than 1.75 inches since these will take up multiple U and greatly increase what you need. However, some smaller pieces of equipment might fit into a U alongside another small piece of equipment to save your space (but be careful not to block the airflow of the rack).

Note: If your current IT system uses UPS to protect against power failures, you don’t need to move these into your server rack since the power supply from the data centre will have UPS and other redundancies already built in.

So, to calculate your rack size you need to add together the numbers for:

  • The servers you have
  • The other pieces of equipment you have which are 1.75 inches tall
  • The U you will need to fit your smaller pieces of equipment into
  • The pieces of equipment that are taller than 1.75 inches, factoring in that they will need an additional U for each 1.75 inches of height.

This will let you know if you need a quarter, half, or full rack.

Other considerations for calculating your rack size

Other than how many pieces of equipment you have, there are some other considerations that might impact what size of rack you need.

1. Security

Taking only a handful of U in a shared colocation service means sharing the rack with other companies. If you need to maintain a certain level of security, you may have to take a quarter rack as a minimum to ensure no one else has access to your equipment.

Additionally, you might want to take a caged rack for maximum security. This means you have a cage around your rack so that other people in the data centre can’t even access the outside of your rack. This is very secure but will typically require you to take a full rack from the data centre and pay some additional fees.

2. Power draw

The power available to draw in a rack depends on its size. If you have a power-hungry IT setup you may have to take a rack that has more space than you need so that the power supply is suitable for your IT.

3. Maintenance

Fully loading a rack is efficient for day-to-day operations, but when it’s time to perform maintenance or if something breaks, you will have an easier time if there is some spare space in the rack. This will allow you to install new servers and get them up and running before removing the old/broken ones. While this might seem like a small thing, business continuity is important, and it might be worth taking a larger rack to help minimise disruption and downtime caused by maintenance.

4. Expansion

Digital transformation is an ongoing process, and the IT system you have today will not be staying the same for long. If it looks like you’ll be expanding your IT system in the near future then you might want to take a larger rack than you currently need to minimise effort when it’s time to grow.

Thankfully some data centres offer a pay-as-you-grow service. With the promise of future expansion from yourself, they will let you take a full rack while only charging you for what you’re currently using. So, if you currently have a half-rack’s worth of equipment but will be expanding at some point, you can keep your equipment in a full rack but only be charged for a half rack until you start to fill it up.

Get advice on your rack size options

If you’re looking to migrate into a data centre to upgrade your IT’s resilience and reliability we’re here to help. Whether you need advice on calculating your rack size, or support with performing a seamless migration, Redcentric is a people-focussed data centre operator with a long history of helping businesses with their digital transformation. If you want to ask some questions with absolutely zero commitment, get in touch with one of our experts.

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