I just upgraded the memory in my 2017 iMac and tests with Geekbench show that the modules should indeed be paired by DIMM slot, not by bank. The difference is significant too. With the same modules in different banks I get a memory bandwidth of 30 GB/s, with the modules in the same bank (but different DIMM slots) memory bandwidth drops to 16 GB/s. Slot-loading model. On October 5, 1999, Apple discontinued the tray-loading iMac. The new iMac built upon the Revision D's success with a faster processor, double the RAM, improvements to the built-in speaker system, a slot-loading optical drive, faster ATI Rage 128 VR graphics, a slightly updated case, and support for Apple's 802.11b AirPort wireless networking card.

Choose your iMac model

If you're not sure which iMac you have, you can identify your iMac and then select it from the list below.

27-inch

24-inch

21.5-inch

• iMac (Retina 4K, 21.5-inch, 2019)*
• iMac (Retina 4K, 21.5-inch, 2017)*
• iMac (21.5-inch, 2017)*
• iMac (21.5 inch, Mid 2014)*
• iMac (21.5-inch, Late 2013)*
• iMac (21.5-inch, Late 2012)*

20-inch

17-inch

* Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017) and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorised Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorised Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorised Service Provider offers memory upgrade services.

Memory in iMac (21.5-inch, Late 2015) and iMac (Retina 4K, 21.5-inch, Late 2015) is not upgradeable.

iMac (Retina 5K, 27-inch, 2020)

Get memory specifications for iMac (Retina 5K, 27-inch, 2020), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

For optimal memory performance, DIMMs should be the same capacity, speed and supplier. Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-21333
• Unbuffered
• Nonparity
• 260-pin
• 2666MHz DDR4 SDRAM

If you have mixed capacity DIMMs, see the install memory section for installation recommendations.

iMac (Retina 5K, 27-inch, 2019)

Get memory specifications for iMac (Retina 5K, 27-inch, 2019), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-21333
• Unbuffered
• Nonparity
• 260-pin
• 2666MHz DDR4 SDRAM

iMac (Retina 5K, 27-inch, 2017)

Get memory specifications for iMac (Retina 5K, 27-inch, 2017), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-2400 (19200)
• Unbuffered
• Nonparity
• 260-pin
• 2400MHz DDR4 SDRAM

iMac (Retina 5K, 27-inch, Late 2015)

Get memory specifications for iMac (Retina 5K, 27-inch, Late 2015), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC3-14900
• Unbuffered
• Nonparity
• 204-pin
• 1867MHz DDR3 SDRAM

For these 27-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (Retina 5K, 27-inch, Mid 2015)
• iMac (Retina 5K, 27-inch, Late 2014)
• iMac (27-inch, Late 2013)
• iMac (27-inch, Late 2012)

Memory specifications

These iMac models feature Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC3-12800
• Unbuffered
• Nonparity
• 204-pin
• 1600MHz DDR3 SDRAM

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Open the memory compartment door by pressing the small grey button located just above the AC power port:
5. The memory compartment door will open as the button is pushed in. Remove the compartment door and set it aside:
6. A diagram on the underside of the compartment door shows the memory cage levers and the orientation of the DIMM. Locate the two levers on the right and left sides of the memory cage. Push the two levers outwards to release the memory cage:
7. After the memory cage has been released, pull the memory cage levers towards you, allowing access to each DIMM slot.
8. Remove a DIMM by pulling the module straight up and out. Note the location of the notch on the bottom of the DIMM. When reinstalling DIMMs, the notch must be orientated correctly or the DIMM won't fully insert:
9. Replace or install a DIMM by setting it down into the slot and pressing firmly until you feel the DIMM click into the slot. When you insert a DIMM, make sure you align the notch on the DIMM to the DIMM slot. Find your model below for specific installation instructions and notch locations:
   • iMac (Retina 5K, 27-inch, 2020) DIMMs have a notch on the bottom, slightly to the left of the centre. If your DIMMs are mixed in capacity, minimise the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible.
     
   • iMac (Retina 5K, 27-inch, 2019) DIMMs have a notch on the bottom, slightly to the left of the centre:
   • iMac (27-inch, Late 2012) and iMac (Retina 5K, 27-inch, 2017) DIMMs have a notch on the bottom left:
     
   • iMac (27-inch, Late 2013) and iMac (Retina 5K, 27-inch, Late 2014, Mid 2015, and Late 2015) DIMMs have a notch on the bottom right:
     
10. After you've installed all of your DIMMs, push both memory cage levers back into the housing until they've locked into place:
    
11. Replace the memory compartment door. You don't need to press the compartment door release button when replacing the compartment door.
12. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.
    

Your iMac performs a memory initialisation procedure when you first turn it on after upgrading memory or rearranging DIMMs. This process can take 30 seconds or more, and the display of your iMac remains dark until it's finished. Make sure you let the memory initialisation complete.

For these 27-inch and 21.5-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

Imac Ram Slot Numbers Chart

• iMac (27-inch, Mid 2011)
• iMac (21.5-inch, Mid 2011)
• iMac (27-inch, Mid 2010)
• iMac (21.5-inch, Mid 2010)
• iMac (27-inch, Late 2009)
• iMac (21.5-inch, Late 2009)

Memory specifications

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

i5 and i7 Quad Core iMac computers come with both top memory slots populated. These computers will not start up if only a single DIMM is installed in any bottom slot; these computers should operate normally with a single DIMM installed in any top slot.

Core Duo iMac computers should operate normally with a single DIMM installed in any slot, top or bottom. ('Top' and 'bottom' slots refer to the orientation of the slots in the pictures below. 'Top' refers to the slots closest to the display; 'bottom' refers to the slots closest to the stand.)

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Philips screwdriver, remove the RAM access door in the bottom of your computer:
   
5. Remove the access door and set it aside.
6. Untuck the tab in the memory compartment. If you're replacing a memory module, gently pull the tab to eject any installed memory module:
   
7. Insert your new or replacement SO-DIMM into the empty slot, noting the orientation of the keyway of the SO-DIMM as shown below.
8. After you've inserted it, press the DIMM up into the slot. There should be a slight click when you seat the memory correctly:
   
9. Tuck the tabs above the memory DIMMs, and reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

Imac Ram Slots

For these 24-inch and 20-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (24-inch, Early 2009)
• iMac (20-inch, Early 2009)
• iMac (24-inch, Early 2008)
• iMac (20-inch, Early 2008)
• iMac (24-inch Mid 2007)
• iMac (20-inch, Mid 2007)

Memory specifications

These iMac computers have two side-by-side Synchronous Dynamic Random-Access Memory (SDRAM) slots in the bottom of the computer.

The maximum amount of random-access memory (RAM) you can install in each computer is:

You can use a 1 GB or 2 GB RAM module in each slot for iMac (Mid 2007) and iMac (Early 2008). Use 1 GB, 2 GB or 4 GB modules in each slot for the iMac (Early 2009).

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

DIMMs with any of the following features aren't supported:

• Registers or buffers
• PLLs
• Error-correcting code (ECC)
• Parity
• Extended data out (EDO) RAM

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After your iMac has cooled down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Philips screwdriver, remove the RAM access door in the bottom of the computer:
   
5. Remove the access door and set it aside.
6. Untuck the tab in the memory compartment. If you're replacing a memory module, untuck the tab and pull it to eject any installed memory module:
   
7. Insert your new or replacement RAM SO-DIMM into the empty slot, noting the orientation of the keyway of the SO-DIMM as shown above.
8. After you've inserted it, press the DIMM up into the slot. There should be a slight click when you seat the memory correctly.
9. Tuck the tabs above the memory DIMMs, and reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

For these 20-inch and 17-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (20-inch Late 2006)
• iMac (17-inch, Late 2006 CD)
• iMac (17-inch, Late 2006)
• iMac (17-inch, Mid 2006)
• iMac (20-inch, Early 2006)
• iMac (17-inch, Early 2006)

Memory specifications

For best performance, fill both memory slots, installing an equal memory module in each slot.
*iMac (Late 2006) uses a maximum of 3GB of RAM.

Installing memory in the bottom slot

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Phillips screwdriver, remove the RAM access door on the bottom of the iMac and set it aside:
   
5. Move the DIMM ejector clips to their fully open position:
   
6. Insert your RAM SO-DIMM into the bottom slot, keeping in mind the orientation of the keyed SO-DIMM:
   
7. After you've inserted it, press the DIMM up into the slot with your thumbs. Don't use the DIMM ejector clips to push in the DIMM, as this may damage the SDRAM DIMM.There should be a slight click when you seat the memory fully.
8. Close the ejector clips:
   
9. Reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

Replacing memory in the top slot

After you've shut down your iMac and given it some time to cool down, follow these steps:

Imac Ram Slot Numbers Diagram

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Phillips screwdriver, remove the RAM access door on the bottom of the iMac and set it aside:
   
5. Pull the two levers on each side of the memory compartment to eject the memory module that's already installed:
   
6. Remove the memory module from your iMac as shown below:
   
7. Insert your RAM SO-DIMM into the top slot, noting the orientation of the keyed SO-DIMM:
   
8. After you've inserted it, press the DIMM up into the slot with your thumbs. Don't use the DIMM ejector clips to push in the DIMM, as this may damage the SDRAM DIMM.There should be a slight click when you seat the memory fully.
9. Close the ejector clips:
   
10. Reinstall the memory access door:
    
11. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

If your iMac makes a tone after you install memory

iMac models introduced before 2017 might make a warning sound when you start up after installing or replacing memory:

• One tone, repeating every five seconds signals that no RAM is installed.
• Three successive tones, then a five-second pause (repeating) signals that RAM doesn't pass a data integrity check.

If you hear these tones, confirm that the memory you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If your Mac continues to make the tone, contact Apple Support.

Confirm that your iMac recognises its new memory

How Much Ram For Imac

After you've installed memory, you should confirm that your iMac recognises the new RAM by choosing Apple () menu > About This Mac.

The window that appears lists the total memory, including the amount of memory that originally came with the computer plus the newly added memory. If all the memory in the iMac has been replaced, it lists the new total of all installed RAM.

For detailed information about the memory installed in your iMac, click System Report. Then choose Memory under the Hardware section in the left side of System Information.

Ram For Imac 27

If your computer doesn't recognise the memory or doesn't start up correctly, confirm that the RAM you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If you continue to have issues with the memory in your iMac, contact Apple Support.