HP EliteBook 840 G3 upgrade guide: RAM, SSD and CPU

Lifecycle and support

A current model: sold new and supported.

Dates not listed haven't been published by the manufacturer or a reliable third party.

RAM upgrade

There are two SO-DIMM sockets, side by side on the bottom of the laptop, and both are user-accessible — no memory is soldered to the board. The machine takes DDR4 SODIMMs running at 2133 MT/s (PC4-17000). HP's spec sheet states a maximum of 32 GB, which means 2 × 16 GB modules.

It's worth knowing that HP's documents disagree here. The spec sheet says 32 GB, but the Maintenance and Service Guide — which covers the 840 G3 alongside the 848 G3, 850 G3 and ZBook 15u G3 — only lists 16 GB modules as "mobile workstation only" and shows 16 GB as a 2 × 8 GB configuration. In practice, 2 × 16 GB kits are widely reported to work in the 840 G3, but they're only formally validated on the ZBook 15u G3 variant. If you want to try 32 GB, buy from a seller that accepts returns.

Use standard unbuffered, non-ECC DDR4 SODIMMs with plain JEDEC timings (CL14-14-14, 2133 or 2400). HP's BIOS has no XMP support, and modules with tighter sub-timings have been known to fail to boot entirely. Faster RAM won't gain you anything beyond 2133 MT/s behaviour here, so don't pay extra for exotic kits. If you fit modules of different sizes or speeds, the system still works but runs single-channel at the slower speed — a matched pair is the way to get dual-channel.

If you install only one module, it must go in the socket labelled "1".

Buying tip: plain JEDEC DDR4-2133 SODIMMs are safest; skip anything advertising XMP profiles or aggressive timings.

Buying tips

• Choose plain JEDEC DDR4-2133 SODIMMs, non-ECC and unbuffered
• A matched pair gives you dual-channel; mixed modules drop to single-channel at the lower speed
• For 32 GB, check the seller's returns policy first

Storage upgrade

You get two drive positions: a 2.5-inch SATA bay and an M.2 2280 slot, and you can fill both at once — unless the machine was built with the hybrid SSHD option, in which case the dual-drive setup isn't available.

The 2.5-inch bay takes SATA-3 drives up to 7.0 mm thick, and that limit is firm: HP's Service Guide is explicit about it. Some compatibility listings suggest 9.5 mm drives fit; they don't — a 9.5 mm drive generally won't let you close the case. All drives use the same mounting bracket or caddy.

The M.2 slot takes 22 × 80 mm (2280) drives in either SATA or PCIe NVMe flavour, with B+M or M key connectors. The slot runs at PCIe 3.0 ×4 at most, so a Gen4 NVMe drive will work but simply run at Gen3 speed — there's no minimum PCIe generation required. Don't pay a premium for Gen4 speed you can't use.

There's no documented capacity limit. HP shipped up to 512 GB M.2 drives and 1 TB hard drives, while larger aftermarket drives — 2 TB 2.5-inch 7 mm SSDs and 1–2 TB NVMe modules — are commonly used but aren't HP-validated, so treat those as a gamble and buy with returns. Also remember that the recovery partition reserves around 16 GB (Windows 7) or up to 30 GB (Windows 8.1/10), so the formatted capacity you see in Windows will be lower than the drive's advertised size.

Buying tip: measure twice on thickness — 7 mm is the hard ceiling for the 2.5-inch bay. For the M.2 slot, a mid-range Gen3 NVMe drive gets you everything the slot can deliver.

Buying tips

• 2.5-inch drives must be 7 mm thick or less — 9.5 mm drives won't fit
• Any 2280 SATA or NVMe drive works; Gen4 drives just run at Gen3 speed
• Bigger than 1 TB (2.5-inch) or 1–2 TB (NVMe) usually works but isn't HP-validated

How to upgrade

Before you start

• Back up your data first — drive and OS changes can go wrong.
• Update the BIOS before installing new RAM. HP warns that skipping this can cause system problems.
• Shut down the laptop, unplug the charger and remove any connected devices.
• Ground yourself (touch a metal object or use an anti-static strap) before handling components.
• Open the case and install the part. Use HP's Maintenance and Service Guide for the exact disassembly steps for your unit — screw and latch placement vary between the regional SKUs.
• Close the case, plug in and power on, then check that the BIOS/firmware and the operating system recognise the new part.

Steps

1. How to install RAM
   Follow the outline above. Pop the modules into the two SO-DIMM sockets at an angle and press them down until the clips click. If you're using a single module, it must go in the socket labelled "1". Then boot and confirm the total in the BIOS or Windows.
2. How to install a drive
   Follow the outline above. Both drive types use the same mounting bracket, so transfer it to the new drive before fitting. Keep the 2.5-inch bay to drives 7 mm thick or less. After booting, check the BIOS sees the drive and initialise/format it in Windows — remembering the recovery partition will claim some space.

Good to know

• HP's documents disagree on 32 GB RAM support (spec sheet says yes, Service Guide validates 16 GB modules only for the ZBook 15u G3).
• Compatibility listings conflict with HP on 2.5-inch drive thickness — HP's 7 mm limit is authoritative.
• Large aftermarket drive capacities are commonly used but not validated by HP.

Frequently asked questions

How much RAM can the HP EliteBook 840 G3 take?

HP's spec sheet says up to 32 GB, using 2 × 16 GB DDR4-2133 SO-DIMMs. Note that the Service Guide only formally validates 16 GB modules for the ZBook 15u G3 variant, while 2 × 16 GB is widely reported to work in the 840 G3 — buy from a seller that accepts returns.

Can you upgrade the CPU in an EliteBook 840 G3?

No. The processor is soldered to the board (6th-gen Skylake-U, i3-6100U up to i7-6600U), so there's no socket and it cannot be replaced.

Does the EliteBook 840 G3 support NVMe SSDs?

Yes. Its M.2 2280 slot accepts both SATA and PCIe NVMe drives (B+M or M key), running at up to PCIe 3.0 ×4. Gen4 drives work but run at Gen3 speed.

How thick a 2.5-inch drive fits in the EliteBook 840 G3?

7.0 mm maximum. Some listings suggest 9.5 mm drives fit, but HP's Service Guide is clear: only 7 mm drives, and thicker ones generally won't let the case close.

Can I run two drives at once?

Yes — a 2.5-inch SATA drive plus an M.2 SSD simultaneously — unless the laptop was built with the hybrid SSHD option, which rules out the dual-drive setup.

Why is my new RAM not booting?

Two common causes. First, the BIOS wasn't updated before the install — HP warns this can cause problems. Second, the modules use aggressive XMP timings; the BIOS has no XMP support, so use plain JEDEC CL14 DDR4-2133/2400 SODIMMs.

Official documents and references

• PDF HP Maintenance and Service Guide — EliteBook 840 G3 / 848 G3 / 850 G3 / ZBook 15u G3 (PDF)
  The definitive source for disassembly, supported parts and the 7 mm drive limit.
• HP EliteBook 840 G3 product specifications
  HP's spec sheet listing memory, storage options and the 32 GB maximum.
• HP EliteBook 840 G3 Notebook PC — product page
  Use this to confirm your exact product number and regional SKU before ordering parts.
• HP support thread: 840 G3 won't boot with XMP-timed RAM
  Real-world example of why plain JEDEC modules matter.
• Notebookcheck review of the HP EliteBook 840 G3
  Independent measurements of CPU power limits and throttling behaviour.