Intel Core i5-8400 vs Intel Core i7-6700K Comparison

The Verdict

The benchmark database presents a fascinating generational clash. The Intel Core i5-8400, a six-core Coffee Lake part from 2017, and the Intel Core i7-6700K, a four-core, eight-thread Skylake flagship from 2015, are separated by two years of silicon evolution. The data shows a clear, albeit nuanced, winner in the i5-8400, which takes 13 of 19 head-to-head tests. The i7-6700K retains a distinct edge in single-threaded and specific integer workloads, but the i5-8400’s extra physical cores give it a commanding lead in most multi-threaded and specialized instruction tests.

For users prioritizing modern multi-core rendering, data compression, or floating-point math, the i5-8400 is the definitive choice. Its wins in Cinebench R23 multi-core (7847 vs. 7594) and PassMark floating point (22006 vs. 17375) reflect a 3.3% and 26.7% advantage, respectively. Conversely, the i7-6700K justifies its existence for legacy single-threaded tasks or workloads that favor its higher clock speed and Hyper-Threading, such as PassMark integer math (28144 vs. 25573, a 9.1% edge). The i5-8400 is the better all-rounder; the i7-6700K is a specialist in older, single-thread-heavy applications.

Architecture Differences

The two processors represent distinct architectural eras. The i5-8400 is built on the Coffee Lake architecture, while the i7-6700K uses Skylake. Both are fabricated on the same 14 nm process node at Intel, but the die sizes differ dramatically: the i5-8400 has a 154 mm² die, whereas the i7-6700K is smaller at 122 mm².

Core configuration is the primary differentiator. The i5-8400 offers 6 physical cores and 6 threads, with no Hyper-Threading. The i7-6700K provides 4 physical cores and 8 threads, leveraging Hyper-Threading to double its logical processor count. This means the i5-8400 has two additional physical cores, while the i7-6700K relies on simultaneous multithreading to fill its worker slots.

Cache hierarchies also diverge. Both share 64 KB of L1 and 256 KB of L2 per core, but the shared L3 cache is 9 MB on the i5-8400 versus 8 MB on the i7-6700K. Clock speeds favor the older chip: the i7-6700K has a 4.00 GHz base and 4.20 GHz boost, while the i5-8400 runs at 2.80 GHz base and 4.00 GHz boost. The i7-6700K also has an unlocked multiplier, making it overclockable, while the i5-8400 is locked.

Memory support and integrated graphics differ as well. The i5-8400 supports DDR4 only, with a 42.7 GB/s memory bandwidth. The i7-6700K supports both DDR3 and DDR4 depending on the motherboard, but its bandwidth is listed at 34.1 GB/s. The i5-8400 integrates UHD Graphics 630, while the i7-6700K has the older HD Graphics 530. Both use the same Intel Socket 1151 and provide PCIe Gen 3 with 16 CPU lanes. Power envelopes are notably different: the i5-8400 has a 65 W TDP, while the i7-6700K draws 91 W.

FAQ

Q: Which processor has more physical cores?

A: The Intel Core i5-8400 has 6 physical cores, while the Intel Core i7-6700K has 4 physical cores. However, the i7-6700K supports 8 threads via Hyper-Threading, whereas the i5-8400 has only 6 threads total.

Q: Is the i7-6700K faster in single-core performance?

A: Yes, in the recorded data. The i7-6700K wins the Geekbench single-core test (1442 vs. 1353, a 6.2% lead) and PassMark single-thread (2500 vs. 2371, a 5.2% lead). Its higher base clock of 4.00 GHz and boost of 4.20 GHz contribute to this advantage.

Q: How does the i5-8400 perform in multi-threaded rendering?

A: It consistently outperforms the i7-6700K. In Cinebench R23 multi-core, the i5-8400 scores 7847 versus 7594, a 3.3% lead. In Cinebench R20 multi-core, it scores 3295 versus 3189, also a 3.3% advantage. The pattern holds across all Cinebench multi-core tests.

Q: Do both processors support ECC memory?

A: No. Neither the Intel Core i5-8400 nor the Intel Core i7-6700K supports ECC memory, according to the database.

Q: Which CPU has a larger L3 cache?

A: The Intel Core i5-8400 has 9 MB of shared L3 cache, while the Intel Core i7-6700K has 8 MB of shared L3 cache.

Q: What is the TDP difference between the two?

A: The i5-8400 has a TDP of 65 W, while the i7-6700K has a TDP of 91 W. The newer chip is more power-efficient on paper, despite having two more physical cores.

Specification Differences

| Specification | Intel Core i5-8400 | Intel Core i7-6700K |

|---|---|---|

| Cores | 6 | 4 |

| Threads | 6 | 8 |

| Base Clock | 2.80 GHz | 4.00 GHz |

| Boost Clock | 4.00 GHz | 4.20 GHz |

| TDP | 65 W | 91 W |

| Architecture | Coffee Lake | Skylake |

| Die Size | 154 mm² | 122 mm² |

| L3 Cache | 9 MB (shared) | 8 MB (shared) |

| Memory Support | DDR4 | DDR3, DDR4 (depends on motherboard) |

| Memory Bandwidth | 42.7 GB/s | 34.1 GB/s |

| Integrated Graphics | UHD Graphics 630 | HD Graphics 530 |

| Multiplier Unlocked | No | Yes |

| Launch MSRP | $182 | $339 |

| Release Date | 2017-10-04 | 2015-08-04 |

Head-to-Head Benchmarks

The i5-8400 dominates the Cinebench suite entirely. In Cinebench R15 multi-core, it scores 790 versus 765, a 3.3% win. Single-core R15 also goes to the i5-8400, 111 versus 108, a 2.8% edge. This is surprising given the i7-6700K’s higher clock, but the database records it as a win for the newer chip. R20 multi-core (3295 vs. 3189) and single-core (464 vs. 450) both favor the i5-8400 by 3.3% and 3.1%, respectively. R23 multi-core (7847 vs. 7594) and single-core (1107 vs. 1072) continue the trend, each with a 3.3% lead.

The i7-6700K strikes back in Geekbench single-core, winning 1442 versus 1353, a 6.2% margin. However, the i5-8400 takes Geekbench multi-core, 5088 versus 5011, a smaller 1.5% win. PassMark results reveal the most dramatic splits. The i5-8400 crushes the i7-6700K in extended instructions, 11543 versus 8248, a staggering 39.9% advantage. Floating-point math also goes heavily to the i5-8400, 22006 versus 17375, a 26.7% lead. Prime number finding favors the i5-8400 by 23.3% (37 vs. 30). Data compression is a 3.4% win for the i5-8400 (129951 vs. 125659), and multithread performance is 3.3% higher (9225 vs. 8927). Random string sorting sees a 1% edge for the i5-8400 (15952 vs. 15788).

The i7-6700K’s wins are concentrated in integer-heavy tasks. PassMark integer math goes to the i7-6700K, 28144 versus 25573, a 9.1% advantage. Data encryption favors the i7-6700K, 3067 versus 2824, a 7.9% lead. PassMark single-thread (2500 vs. 2371) and its duplicate singlethread test show a 5.2% edge for the older chip. The closest contest is PassMark physics, where the i7-6700K wins by a razor-thin 0.3% (637 vs. 635).

Where Each One Wins

The Intel Core i5-8400 is the clear victor in modern multi-threaded workloads. Its six physical cores provide a structural advantage that Hyper-Threading on the i7-6700K cannot fully offset. This shows in every Cinebench multi-core test and in PassMark multithread, data compression, and random string sorting. The i5-8400 also excels in specialized instruction execution, with a massive 39.9% lead in extended instructions, suggesting better handling of new instruction sets. Floating-point math and prime number computation are also heavily in its favor, making it the superior choice for scientific computing, rendering, and modern game physics that leverage multiple cores.

The Intel Core i7-6700K remains relevant for single-threaded and specific integer workloads. Its higher base and boost clocks deliver wins in Geekbench single-core, PassMark single-thread, and PassMark integer math. The data encryption win is notable, suggesting that its Hyper-Threading implementation and older architecture handle certain cryptographic operations more efficiently. The physics test is a statistical tie, with a 0.3% difference, meaning either chip performs equivalently in that specific benchmark.

For a buyer today, the i5-8400 offers broader applicability across the recorded benchmarks. The i7-6700K, while still competitive in legacy single-threaded applications, is increasingly niche. The data implies that for anyone running modern software that scales across cores, the i5-8400 is the more future-proof choice, despite its lower clock speeds and lack of overclocking. The i7-6700K’s only clear territory is older software or workloads that are strictly single-threaded, where its clock advantage and unlocked multiplier provide tangible benefits.