A six-decade resistance box covering 0.01 Ω to 11.1111 MΩ with sub-6 mΩ zero resistance and silver-alloy contacts is the typical lab-grade configuration, exemplified by the IET Labs GenRad 1433-series platform [S1]. A seven-decade design, exemplified by the GME RD-300 listed at 1 Ω to 11 MΩ with 1% accuracy and 1 W resistors, sacrifices the lowest decade to gain an extra order of magnitude at the top or bottom [S2]. The decision hinges on whether the application needs 0.01 Ω resolution or 11.1111 MΩ end-point coverage, because no single off-the-shelf unit economically delivers both with high accuracy.
Decade count alone does not determine usability: per-decade accuracy, switch contact resistance, and per-resistor power rating matter more for most production test work. For a refresher on how stacked series resistors create additive values, the decade resistance box principle is straightforward, and a box's actual performance at low ohms is dominated by switch contact resistance rather than the resistor tolerances [S8].
Range, decade count, and what the numbers actually mean
A 6-decade box from 0.01 Ω to 11.1111 MΩ exposes six switches: 0.01 Ω, 0.1 Ω, 1 Ω, 10 Ω, 100 Ω, 1 kΩ, 10 kΩ, 100 kΩ, 1 MΩ steps combined additively, giving a maximum of 11,111,110.1 Ω in 0.01 Ω increments [S1]. IET Labs' GenRad 1433 line reaches 1 mΩ to 111.11111111 MΩ across 3 to 11 dials in 36 models, while the HARS series spans 1 mΩ to 111 MΩ in single through 11-decade units at 0.01% (100 ppm) or 0.1% accuracy [S1].
A 7-decade box running 1 Ω to 11 MΩ such as the GME RD-300 (1% accuracy, 1 W resistors) trades the 0.01 Ω and 0.1 Ω dials for one extra high-end decade, accepting 1 Ω as the minimum step [S2]. The EISCO 6-decade unit covers 0 to 1,111,110 Ω in 1 Ω steps, and the Time Electronics 1051 covers 0.01 Ω to 1 MΩ across 8 thumbwheel digits with metal-film resistors and gold contacts [S2][S5]. Counting the digits is not the same as counting decades: the 1051 is effectively 8 decades from 0.01 Ω through 100 kΩ, reaching 1 MΩ only at the maximum setting.
Per-decade accuracy comparison across common configurations
Accuracy is not uniform across decades. The Time Electronics 1051 publishes per-decade accuracy that steps from ±10% on the 0.01 Ω dial down to ±0.1% on the 1 kΩ and higher dials, reflecting that low-value resistors and contact resistance dominate the error budget at the bottom of the range [S5]. IET Labs' HARS series locks 0.01% (100 ppm) or 0.1% across the whole range, while the high-power HPRS series quotes 0.5% or 1% with 225 W per decade and 250 W maximum [S1].
Comparing the three common configurations side by side, a 6-decade 0.01 Ω to 11.1111 MΩ box (IET GenRad-class) gives the widest range with sub-6 mΩ zero resistance and silver-alloy contacts but costs lab-grade money [S1]. A 7-decade 1 Ω to 11 MΩ box (GME RD-300 class) is the budget choice at roughly $80 with 1% accuracy and 1 W resistors, and adds an extra high-end decade at the cost of sub-ohm resolution [S2]. An 8-decade 0.01 Ω to 1 MΩ box (Time Electronics 1051 class) fits mid-range at compact size with gold contacts, per-decade accuracy from ±10% to ±0.1%, and Pt100 simulation capability, but stops at 1 MΩ [S5].
Switch contacts, zero resistance, and the sub-ohm problem
At 0.01 Ω to 0.1 Ω dial settings, the switch contact resistance can exceed the selected resistor value, so published "zero resistance" is a real spec. IET Labs specifies the GenRad 1433 platform with solid silver-alloy contacts and zero resistance below 6 mΩ, which is what allows the 0.01 Ω decade to be meaningful [S1]. The Time Electronics 1051 uses a multiple gold-contact arrangement claimed to keep contact resistance low and to tolerate single-contact failure [S5]. Cheap 7-decade substitution boxes typically do not publish zero resistance, which is why 0.01 Ω resolution on a $25 kit is marketing, not measurement [S2].
For insulation resistance testing workflows that need values up into the gigaohm or teraohm range, none of the 0.01 Ω to 11 MΩ boxes apply. IET's HRRS-5 kV and 10 kV series reach 1 GΩ to 11 TΩ at 5,000 V or 10,000 V maximum, and the VRS-100 series ships fixed resistor sets at 20 ppm accuracy up to 10 TΩ for Megger calibration [S1]. Decade box selection should match the measurement, not the catalog headline.
Power rating, voltage, and thermal behaviour
Per-resistor power rating determines what current the box can sink. The GME RD-300 specifies 1 W resistors across all seven decades, so at 1 Ω the box can handle roughly 1 A continuously, dropping to 0.03 A at 1 MΩ [S2]. IET's HPRS high-power line rates 225 W per decade with 250 W maximum for applications where standard 1 W boxes would derate severely [S1]. The Time Electronics 1051 does not publish a per-decade wattage but is designed around low-power metal-film resistors typical of 0.5 W to 1 W per element [S5].
Voltage limits matter when the selected resistance is high. A 7-decade DIY builder on the PedalPCB forum was undecided on maximum voltage rating while targeting 2 W per element, illustrating that published wattage and the actual voltage the switch insulation can withstand are separate constraints [S4]. A 1 MΩ setting at 100 V dissipates only 10 mW but stresses the switch at 100 V; the 1051 uses safety terminals with limited opening distance and a touch-proof raised flange to address this [S5].
Use-case fit: 6-decade vs 7-decade for specific jobs
For RTD simulation around the Pt100 100 Ω to 400 Ω range with 0.01 Ω steps, a 6-decade 0.01 Ω to 11.1111 MΩ box hits the sweet spot: low zero resistance, mid-range accuracy at 0.1% or better, and resolution that exceeds Pt100 class A tolerance [S1][S5]. The IET OS-260 and OS-270 are explicitly engineered as RTD calibrators with 0.01% accuracy and 0.01 Ω resolution, and cannot be computer-controlled [S1].
For general breadboard and audio work where the budget is under $100, a 7-decade 1 Ω to 11 MΩ box at 1% accuracy is the practical choice: it covers the 10 Ω to 1 MΩ range used in 99% of analog and digital prototyping with acceptable error and easy sourcing [S2][S4]. An EEVblog builder aiming for seven decades with sub-ohm total error found that off-the-shelf 1% resistors combined in two-series-plus-parallel trim networks held fractional-ohm error up to 100 kΩ but degraded above 1 MΩ where leakage paths dominated, a limitation inherent to the topology rather than the parts [S3].
Selection checklist and what to verify before buying
Before specifying, confirm five numbers: full-scale maximum resistance, minimum step (true decade count), per-decade accuracy table, zero resistance of the switches, and per-resistor power rating. A box that reads "0.01 Ω to 11 MΩ" with one published accuracy figure is hiding the worst decade, which is almost always the 0.01 Ω or 0.1 Ω dial [S1][S5]. For Pt100 work, a metal-film resistor box with gold or silver-alloy contacts and at least 0.1% mid-range accuracy is the minimum credible spec; for insulation resistance above 1 GΩ, a separate high-voltage decade or fixed standard is required, not the same box pushed past its design range [S1].
The 7-decade question collapses once the application's minimum required step is fixed. If 1 Ω resolution is acceptable and the budget is under $100, the 7-decade 1 Ω to 11 MΩ format wins. If 0.01 Ω resolution, sub-6 mΩ zero resistance, or MΩ-end accuracy is on the spec, the 6-decade 0.01 Ω to 11.1111 MΩ lab-grade format is the only honest answer. Track the IET HARS-X2 double-power variant and Time Electronics 9026 carry case as near-term availability signals when sizing lab spares [S1][S5].
This topic is covered further in ASTM B822 Light-Scattering PSD: Scope, Equivalence to ISO 13320, and 2025 Update.
