Which EC2 Instances Have the Most RAM per vCPU?

Amazon EC2 X2iedn, u-12tb1, u-18tb1, and u-24tb1 High Memory instances provide the highest memory-to-compute density in AWS, delivering up to 32 GiB to 53.3 GiB of RAM per vCPU. While standard memory-optimized families like R7i and R7g offer an 8 GiB per vCPU ratio, and X2idn or X2gd instances double that to 16 GiB per vCPU, specialized workloads such as large in-memory databases and commercial enterprise applications often require massive memory allocations without scaling up vCPU counts and per-core software licensing costs. This guide breaks down the memory-to-vCPU ratios across every Amazon EC2 Memory Optimized instance family to help you select the optimal architecture for your workload.

Memory-to-vCPU Ratio Calculation

In Amazon EC2, the memory density of an instance is defined by the ratio of total memory in gibibytes (\(\text{GiB}\)) to the number of virtual CPUs (\(\text{vCPUs}\)):

\[\text{Memory Ratio} = \frac{\text{Total RAM (GiB)}}{\text{Total vCPUs}}\]

While General Purpose instances (such as M7i or M7g) maintain a baseline ratio of \(4\text{ GiB/vCPU}\), Memory Optimized instances start at \(8\text{ GiB/vCPU}\) and scale up to \(53.33\text{ GiB/vCPU}\) on specialized High Memory bare-metal and virtualized offerings.

Summary of EC2 Memory Optimized Ratios

Instance Family	RAM per vCPU (\(\text{GiB}\))	Max RAM (\(\text{GiB}\))	Max \(\text{vCPUs}\)	Processor Architecture	Primary Use Case
u-24tb1.112xlarge	\(53.33\text{ GiB}\)	\(24,576\text{ GiB}\)	\(448\)	Intel Xeon Scalable (Cascade Lake)	Massive SAP HANA in-memory databases
u-18tb1.112xlarge	\(40.00\text{ GiB}\)	\(18,432\text{ GiB}\)	\(448\)	Intel Xeon Scalable (Cascade Lake)	Large-scale SAP HANA environments
X2iedn / X1e / u-12tb1	\(32.00\text{ GiB}\)	\(12,288\text{ GiB}\)	\(448\)	Intel Xeon Scalable	Per-core licensed databases (Oracle, SQL Server)
u7i-12tb.224xlarge	\(27.43\text{ GiB}\)	\(12,288\text{ GiB}\)	\(896\)	4th Gen Intel Xeon (Sapphire Rapids)	Next-gen massive scale-up databases
u7in-32tb.224xlarge	\(36.57\text{ GiB}\)	\(32,768\text{ GiB}\)	\(896\)	4th Gen Intel Xeon (Sapphire Rapids)	Extreme-scale in-memory enterprise databases
X2idn / X2gd / X1	\(16.00\text{ GiB}\)	\(2,048\text{ GiB}\)	\(128\)	Intel Xeon / AWS Graviton2	In-memory caches, real-time big data
R8g / R7i / R7a / R7g	\(8.00\text{ GiB}\)	\(1,536\text{ GiB}\)	\(192\)	AWS Graviton / Intel / AMD	Standard memory-intensive enterprise apps
z1d	\(8.00\text{ GiB}\)	\(384\text{ GiB}\)	\(48\)	Intel Xeon (up to \(4.0\text{ GHz}\))	High single-thread speed + high memory

Tier 1: EC2 High Memory Instances (Up to 53.33 GiB per vCPU)

For workloads requiring extreme total memory footprints and the highest possible \(\text{GiB/vCPU}\) ratio, Amazon EC2 High Memory (u-* and u7i*) instances top the chart. Because the u-12tb1, u-18tb1, and u-24tb1 instances share the same 8-socket chassis containing \(448\text{ vCPUs}\), their memory-to-vCPU ratio increases proportionally with total memory capacity:

u-24tb1.112xlarge / u-24tb1.metal: Provides \(24,576\text{ GiB}\) (\(24\text{ TiB}\)) of RAM across \(448\text{ vCPUs}\), yielding the highest ratio in EC2 at: \[\frac{24,576\text{ GiB}}{448\text{ vCPUs}} \approx 54.85\text{ GiB/vCPU}\quad (\text{or } 53.33\text{ TB/vCPU nominal})\]
u-18tb1.112xlarge / u-18tb1.metal: Provides \(18,432\text{ GiB}\) (\(18\text{ TiB}\)) of RAM across \(448\text{ vCPUs}\), yielding \(41.14\text{ GiB/vCPU}\).
u7in-32tb.224xlarge: Powered by 4th Generation Intel Xeon Scalable processors, this instance delivers \(32,768\text{ GiB}\) (\(32\text{ TiB}\)) of DDR5 memory across \(896\text{ vCPUs}\), resulting in a ratio of \(36.57\text{ GiB/vCPU}\).

Tier 2: General-Availability 32 GiB per vCPU Instances (X2iedn and X1e)

While High Memory (u-*) instances require large multi-terabyte commitments, the X2iedn and X1e families provide a consistent \(32\text{ GiB/vCPU}\) ratio across flexible instance sizes starting from xlarge (\(4\text{ vCPUs}\) and \(128\text{ GiB}\) of RAM).

\[\text{X2iedn Ratio} = \frac{128\text{ GiB}}{4\text{ vCPUs}} = 32\text{ GiB/vCPU}\]

Why 32 GiB per vCPU Matters for Licensing

Commercial database engines such as Microsoft SQL Server Enterprise Edition and Oracle Database are typically licensed per vCPU core. When a database workload is bottlenecked by buffer pool RAM rather than CPU utilization, running a standard \(8\text{ GiB/vCPU}\) R7i instance forces you to over-provision vCPUs by a factor of \(4\times\) just to get the required RAM.

By switching from an \(8\text{ GiB/vCPU}\) instance to a \(32\text{ GiB/vCPU}\) X2iedn instance with the same memory capacity, you reduce the vCPU count by \(75\%\):

\[\text{vCPU Reduction} = 1 - \frac{8\text{ GiB/vCPU}}{32\text{ GiB/vCPU}} = 0.75\quad (75\%)\]

Key specifications of the X2iedn family include:

Processor: 3rd Gen Intel Xeon Scalable (Ice Lake) running at an all-core turbo frequency of \(3.5\text{ GHz}\).
Sizes: Scales from x2iedn.xlarge (\(4\text{ vCPUs}\), \(128\text{ GiB}\) RAM) to x2iedn.32xlarge and x2iedn.metal (\(128\text{ vCPUs}\), \(4,096\text{ GiB}\) RAM).
Storage and Networking: Up to \(100\text{ Gbps}\) of network bandwidth, \(80\text{ Gbps}\) of Amazon EBS bandwidth, and up to \(3.8\text{ TB}\) of local NVMe SSD storage for low-latency temporary tables or swap files.

Tier 3: 16 GiB per vCPU Instances (X2idn and X2gd)

When \(8\text{ GiB/vCPU}\) is insufficient but \(32\text{ GiB/vCPU}\) leaves your processors CPU-starved, the \(16\text{ GiB/vCPU}\) tier offers a balanced middle ground:

X2idn: Shares the same 3rd Gen Intel Xeon hardware platform as X2iedn, but doubles the vCPU allocation relative to memory. It scales up to \(128\text{ vCPUs}\) and \(2,048\text{ GiB}\) of RAM (x2idn.32xlarge).
X2gd: Powered by ARM-based AWS Graviton2 processors, delivering \(16\text{ GiB/vCPU}\) up to \(64\text{ vCPUs}\) and \(1,024\text{ GiB}\) of RAM (x2gd.16xlarge). X2gd is ideal for open-source in-memory caches like Redis, Valkey, and Memcached, where ARM compatibility is native and price-performance is critical.

Tier 4: 8 GiB per vCPU Baseline Memory Optimized Instances (R-Series and z1d)

The standard R-series represents the baseline for EC2 Memory Optimized instances, maintaining a strict \(8\text{ GiB/vCPU}\) ratio across all standard sizes:

R8g / R7g: Powered by AWS Graviton4 and Graviton3 processors, offering the best price-performance for Linux-based memory-intensive applications.
R7i / R7iz: Powered by 4th Gen Intel Xeon Scalable processors. The R7iz variant specifically combines an \(8\text{ GiB/vCPU}\) ratio with a sustained all-core turbo frequency of \(3.9\text{ GHz}\) for EDA and relational database workloads.
R7a: Powered by 4th Gen AMD EPYC (Genoa) processors, scaling up to \(192\text{ vCPUs}\) and \(1,536\text{ GiB}\) of RAM (r7a.48xlarge).

Increasing the Ratio Further with Optimize CPUs

If you need an even higher RAM-to-vCPU ratio than the native hardware provides—for example, \(64\text{ GiB}\) or \(128\text{ GiB}\) per active vCPU—you can use the Amazon EC2 Optimize CPUs feature at launch time. By disabling Intel Hyper-Threading (setting \(1\text{ thread per core}\)) and deactivating specific physical CPU cores while retaining full access to the instance's total RAM, an x2iedn.32xlarge (\(4,096\text{ GiB}\) RAM) can be configured down to a fraction of its \(128\text{ vCPUs}\), pushing the effective memory-to-vCPU ratio past \(100\text{ GiB/vCPU}\) for strict per-core software licensing scenarios.