Computed

Gemma 3 12B Instruct on a Radeon RX 7900 XTX

Yes, quantized. Q8_0 is the largest that fits, leaving room for 128k tokens of context.

Change the numbers →Gemma 3 12B Instruct on every card Radeon RX 7900 XTX across every model

Verdict

Q8_0

22.1 GiB usable of 24 GB

Weights (Q4_K_M)

6.9 GiB

22.7 GiB at FP16 · 12.2B params · 1 of 6 quant rows are measured files

KV cache

384.0 KiB per token at FP16

48 layers × 8 KV heads × 256 dimensions

Speed ceiling

69 tok/s

960 GB/s ÷ bytes read per token

Every quant, against this card

Weight bytes are the size of the real published file wherever one exists — 1 of these6 rows are measured from the published checkpoint, the rest computed from the parameter count. Max context is what the KV cache can grow to in whatever memory the weights leave behind, capped at the 128k tokens this model was trained to address.

Bars are the weight bytes at each precision; the dashed line is the usable memory of a Radeon RX 7900 XTX. Drawn from the computed byte counts, not sketched.

Precision	Bits/weight	Weights	Source	Fits	Max context	Ceiling
FP16 / BF16	16.00	22.7 GiB	measured file	−636 MiB	—	23.9 tok/s offloaded
Q8_0	8.50	12.1 GiB	nominal	yes	131,072 (model cap)	69 tok/s
Q6_K	6.56	9.3 GiB	nominal	yes	131,072 (model cap)	88 tok/s
Q5_K_M	5.67	8.0 GiB	nominal	yes	131,072 (model cap)	101 tok/s
Q4_K_M	4.83	6.9 GiB	nominal	yes	131,072 (model cap)	117 tok/s
Q3_K_M	3.91	5.5 GiB	nominal	yes	131,072 (model cap)	141 tok/s

What the context actually costs

The KV cache is 2 · layers · kv_heads · head_dim bytes per token per element — 2 · 48 · 8 · 256 · 2 B = 384.0 KiB. It depends on the key/value head count, not the attention-head count and not the parameter count. This model shares 8 KV heads across 16 query heads, which divides the cache by 2 against multi-head attention.

Sliding-window attention. 40 of this model's 48 layers attend to a 1,024-token window and stop growing there; only the remaining 8 keep scaling with context. That is why the cache figures below flatten out — and why a calculator that ignores the layer pattern over-states this model's long-context footprint several times over.

Context	KV cache (FP16)	KV cache (8-bit)	Plus Q4_K_M weights
4,096	576 MiB	288 MiB	7.4 GiB
8,192	832 MiB	416 MiB	7.7 GiB
32,768	2.3 GiB	1.2 GiB	9.2 GiB
131,072	8.3 GiB	4.2 GiB	15.2 GiB

The speed ceiling, and where it comes from

Generating one token reads every active weight from memory once. At Q8_0 that is 12.1 GiB, plus a pass over the KV cache. A Radeon RX 7900 XTX moves 960 GB/s (384-bit × 20 Gbps), so the arithmetic ceiling is 69 tok/s. Treat it as a bound, not an estimate: attention overhead, kernel launches and imperfect memory access keep real runtimes at roughly 60–80% of it, and nothing pushes past it.

Where these numbers come from

The model

Parameters: 12,187,325,040
Layers: 48
Attention / KV heads: 16 / 8
Head dimension: 256
Trained context: 131,072
Checkpoint as published: 22.7 GiB

Read from unsloth/gemma-3-12b-it — an ungated mirror of google/gemma-3-12b-it, whose own config cannot be fetched without an access token. The parameter count is the Hub's own total over the tensor shapes, not a figure taken from the model's name.

The accelerator

Memory: 24 GB GDDR6
Bandwidth: 960 GB/s
Bus: 384-bit × 20 Gbps
Assumed usable: 92% → 22.1 GiB

Capacity and bandwidth from the vendor's specification. The bandwidth figure is checked against the bus width and data rate it derives from. The usable fraction is an assumption, not a spec: a driver context, compute workspace and any attached display come out of the same pool before a weight is loaded.

Questions this pairing answers

How much VRAM does Gemma 3 12B Instruct need?

22.7 GiB for the weights at FP16 — 12.2B parameters at two bytes each — and 6.9 GiB at Q4_K_M. The KV cache is on top of that and is not a fixed number: this model spends 384.0 KiB per token of context on its full-attention layers, so 8,192 tokens costs a further 832 MiB. A Radeon RX 7900 XTX makes 22.1 GiB of its 24 GB available on the assumption below.

Can a Radeon RX 7900 XTX run Gemma 3 12B Instruct?

Yes, quantized. Q8_0 is the largest that fits, leaving room for 128k tokens of context. That is the weights and the KV cache together, against 22.1 GiB of usable memory.

How fast will Gemma 3 12B Instruct run on a Radeon RX 7900 XTX?

No faster than 69 tokens/second at Q8_0, and in practice below it. Decoding is memory-bound: every token reads all 12.1 GiB of weights plus the cache, and this card moves 960 GB/s. That division is the ceiling — no kernel, runtime or driver beats it, and a real runtime typically reaches 60–80% of it.

Why does the context length change how much memory Gemma 3 12B Instruct needs?

Because the KV cache holds one key and one value vector per token, per layer, for the whole conversation, and it is allocated separately from the weights. This model has 48 layers and 8 key/value heads of 256 dimensions, shared across 16 query heads — grouped-query attention, which divides the cache by 2. That works out at 384.0 KiB per token, except that 40 of the 48 layers use a 1024-token sliding window and stop growing there. Parameter count tells you nothing about this number.

The same model on a different card

A different model on the same card

All 62 models on Radeon RX 7900 XTX →

Method and limits. Weight bytes are the byte size of the real published file wherever one exists, and the model's exact parameter count times the published llama.cpp bits-per-weight where it does not. That distinction is on every row above and it matters at both ends: a sub-1B model's Q4_K_M file runs a third larger than the nominal figure because k-quants keep its embedding tables at higher precision, and an already-4-bit release cannot be quantized upward at all. The KV cache is 2 · layers · kv_heads · head_dim · bytes per token, summed over layers with each sliding-window layer capped at its window. The speed figure is a roofline bound, not a benchmark: bandwidth divided by bytes read per token, which no runtime exceeds and every runtime falls short of. The usable fraction of card memory is an assumption: 92% here, which is what this lane assumes for a dedicated card — the other class assumes 75%, so it is not one number applied to every device. That is the only assumed input on this page; every other figure is computed from the model config and the card's published specification. Nothing on this page is written by a language model. Architecture from the model's published config, fetched 2026-08-07. This checkpoint also carries a vision tower; its parameters are inside the totals above because they load whether or not you send it an image.