Index / MR42

MR42

MR42 — mercuric oxide, 1.4 V, 11.6 x 3.6 mm. Not listed by Energizer, and what still fits does not match the circuit: SR42 drops straight in, and runs 1.55 V — +10.7%, which reads high.

What fits the compartment, and what does not

Fit answers whether it enters the compartment. Voltage answers whether the circuit will notice. A cell that fits is not always a safe substitute.

One cell is the same size, and it is not the same voltage: SR42 runs +10.7% against this one, high enough that a calibrated instrument reads above true. Whether that matters is a question about the device, not about the cell.

Also stamped 1154M, 343, 1151M, 387, 1116M, E400N

Unlisted, and what fits runs at the wrong volts

Energizer no longer lists MR42 (also 1154M), and what fits the compartment does not match the circuit: SR42 drops straight in but runs 10.7% off, high enough that a calibrated instrument reads above true. In a meter or a camera calibrated for this cell, that shows up as a reading, not as a failure.

What the chemistry changes

Everything above answers size and voltage. What it cannot answer is chemistry, and 3 other kinds are in play here beside the mercuric oxide this cell uses.

• Mercuric oxide held a very flat 1.4 V, which is what old meters were calibrated against. The chemistry is banned for consumer sale in the United States and the European Union, and this site reads one maker's catalog, so what still reproduces that voltage is a question it cannot answer.
• Silver oxide holds its voltage almost flat until it is spent, which is why watches and light meters were built around it.
• Alkaline drops in voltage steadily as it empties, so a device calibrated for a flat discharge will drift as it goes.
• Zinc air takes oxygen from the air through the holes in its face. It starts discharging the moment the tab comes off and runs down in weeks whether the device is used or not, and a sealed compartment starves it.

The instruments this cell was calibrated into

Mercuric oxide was chosen for one property: it held its voltage almost flat until it died, so a light meter or an exposure circuit could be calibrated against it and stay calibrated. MR42 is a 1.4 V cell, and the chemistry is banned for consumer sale in the United States and the European Union.

The closest cell in the Energizer data this site reads is SR42 at 1.55 V, which is +10.7% away. On a meter that is not a rounding error, it is the exposure. Zinc air comes closest in voltage and is still made: PR41, PR43 and PR44 run 1.45 V. It is a different envelope and a different discharge life, and it is the direction the photographic trade took.

• A cell that reads off does not announce itself. The needle moves, the numbers look plausible, and the exposure is wrong by a fixed amount every time.
• 10 cells measure close enough to compare against it in the Energizer data this site reads, and 10 of them are still listed.
• The nearest is SR42: it drops straight in and runs 1.55 V, reads off.
• By stored energy it stands 1 of 2 in its envelope, at 168 milliwatt-hours.

Drawn to scale

Every published figure

11 figures are published, and one is not: impedance. The row is absent rather than estimated. It reached shelves as 343, 387 and E400N.

The maker’s records filed under this designation do not all publish the same outside size. The table prints the figure most of them carry; the rest publish a height of 3.5 mm under 1154M and 343. Where the records part company the package in your hand decides, and this page cannot tell you which record it came from.

2 rows in the table are not the maker's but ours: stored energy and energy per cubic centimeter. Everything else is reproduced without change.

What the datasheet does not say

Beyond size and voltage the sheet for MR42 is thin: 2 published figures below, and no impedance, no cutoff and no operating range at all. That silence is the maker's, not ours.

The part occupies 0.3 cubic centimeters and weighs 1.4 g. Capacity and voltage together put 168 milliwatt-hours in that space. Energizer files it for North America.

Mass against volume

1.4 g filling 0.3 cubic centimeters. When the printing has worn off, a kitchen scale separates two cells of one envelope faster than a caliper does.

Where the part is filed

North America. That is a statement about the maker's distribution and not about where the cell works.

Is that a lot of energy?

Nothing else measured in this shell holds more. At 168 milliwatt-hours it leads 1 other cell, and the next one down carries 92% of what it does. Per unit of volume that is 560 mWh per cubic centimeter, denser than 93% of everything measured here.

More energy per cubic centimeter than most

MR42 packs more into its volume than most of what is measured here: 560 milliwatt-hours per cubic centimeter, above 93 percent of the corpus.

10 of those ones are still listed. Its 560 milliwatt-hours per cubic centimeter sit above 93 percent of everything here that publishes both a capacity and a volume. 14 of the 82 coins share its 1.4 V exactly.

• SR48 — the next step down by volume: 0.26 cubic centimeters against this cell's 0.3.
• CR1616 — the next step up: 0.32 cubic centimeters, 107% of this one.
• 1177SO — 11.6 x 1.6 mm, 1.55 V, silver oxide.
• CR11108 — 11.6 x 10.8 mm, 3 V, lithium.

The maker's own part numbers

MR42 carries 6 names beyond the designation, and every one of them is a manufacturer part number: a catalog line, not a size and not a standard.

• 1154M — a manufacturer part number for MR42, not a standard designation.
• 343 — a manufacturer part number for MR42, not a standard designation.
• 1151M — a manufacturer part number for MR42, not a standard designation.
• The maker's own catalog carries 3 product lines under it.

The cell that differs only in depth

The code says the size, so the cells that get confused with MR42 are the ones that keep its diameter and change its depth. 18 cells share this diameter at depths from 1.6 to 10.8 mm, and every one of them enters the same opening.

It stands 1 of 2 in this envelope by stored energy. On a scale the difference shows before it shows on a caliper: this one weighs 1.4 g.

• 1177SO — same diameter, 1.6 mm tall against this cell's 3.6: 2 mm shallower, which neither designation carries.
• 365 — same diameter, 1.6 mm tall against this cell's 3.6: 2 mm shallower, which neither designation carries.
• SR55 — same diameter, 2 mm tall against this cell's 3.6: 1.6 mm shallower, which neither designation carries.
• SR42 — the same 11.6 x 3.6 mm in silver oxide at 1.55 V. Identical on a caliper, different in the circuit.
• BR1225 — 12.5 mm across at 3 V against 11.6 mm here, 0.9 mm apart: too wide to enter in this holder.

What this page cannot tell you

3 of the 7 fields this site reads are empty for MR42: cutoff voltage, impedance and operating temperature. Everything computed from them is absent from this page rather than estimated.

The blanks are the same in all 3 records behind this designation, so they are the catalog's silence and not a merge losing a value. What is not missing is the envelope: 11.6 x 3.6 mm, published and checked against the designation itself.

• No cutoff voltage — so this page cannot tell you what the capacity figure was counted down to, without which the capacity is a number without a scale.
• No impedance — so this page cannot tell you whether the cell can drive a pulse, which is the whole question in a watch or a sensor.
• No operating temperature — so this page cannot tell you whether the maker claims anything at all below freezing.

Before you swap anything

Everything here is computed from published dimensions and nominal voltages. That answers whether a cell enters the compartment and whether the voltage matches on paper. It does not answer what your device does with the difference, and it cannot: a cell that fits can still be the wrong cell.

One more limit, and it is the biggest. This site reads a single manufacturer’s catalog. When a page says a designation is not listed, that means Energizer no longer offers a part under it — not that nobody makes the cell. Other makers may still sell it, and for common sizes they usually do.

A voltage difference is one relation between two cells, and this page prints it from the cell it is about — in every table on the page, including the one listing cells no longer made. A figure always belongs to the cell whose voltage is printed beside it: a cell that runs lower than this one carries a minus here, one that runs higher carries a plus, and two cells at the same nominal voltage carry zero. Meet the same pair on the other cell’s page and the sign is the other one, with the figure taken against that cell instead of this one; where the two run at the same voltage, both pages print zero. The class beside the figure is not read off that figure directly. It is decided on the gap itself, taken against the higher of the two nominal voltages, so one pair of cells gets one class whichever page you meet it on: counted from this side the boundary of the class that still runs falls at 20.0% below and 25.0% above, which is the same gap divided once by the larger nominal and once by the smaller. What genuinely differs between the two directions is the consequence, and the words carry it: a cell that runs low makes a calibrated instrument read below true, and a cell that runs high makes it read above.

3 things this site cannot see. Whether the contacts reach a shorter cell. Whether the device was calibrated for a chemistry that holds its voltage flat, which silver oxide does and alkaline does not. Whether the equipment tolerates the higher current a different chemistry can deliver. Rechargeable cells are never a drop-in for primary cells regardless of size.

Cells measured next to this one

1 cell sits at exactly this size, and 18 more share the diameter at other depths. The first group is interchangeable by measurement; the second is not.

• SR42 — 11.6 x 3.6 mm, 1.55 V
• 1177SO — 11.6 x 1.6 mm, 1.55 V
• 365 — 11.6 x 1.6 mm, 1.55 V
• SR55 — 11.6 x 2 mm, 1.55 V
• LR55 — 11.6 x 2.1 mm, 1.5 V

Where these numbers come from

Dimensions, voltages, chemistries and capacities are published by the manufacturer and reproduced without change. Everything else on the page is ours, computed from those figures: whether one cell fits where another sat, how far apart their voltages are, stored energy and energy per cubic centimeter, the rank within a shell, and the check of the designation against the measured size.

Fit is decided within 0.3 mm on every axis, which is the order of a contact spring's travel. Voltage classes are decided by consequence, not by roundness, and they are decided on the gap between two nominal voltages taken against the higher of the two: 2.0% or less of it is the same working voltage, 6.0% starts to matter to a calibrated instrument, and past 20.0% it is a different class altogether. Measuring the gap against the higher figure is what makes the verdict the same from either cell’s page, while the printed percentage stays counted from the cell you are reading about. Cells with identical stored energy share a rank instead of being ordered arbitrarily.

Ranking uses capacity times nominal voltage, because capacity alone is not comparable across chemistries: 150 mAh at 3 V is twice the energy of 150 mAh at 1.5 V. Neighbors are chosen by measurement and never alphabetically, because two cells filed next to each other by name usually have nothing in common. A blank in any table means the maker publishes no figure, which is not the same as zero.

Source: Energizer technical data, retrieved 1 September 2026.

The records behind this page

MR42 is not one catalog entry. 3 records carry this designation, 2 of them with a technical data sheet, and each is numbered below with a direct link to the sheet the figures came from.

1. [1] Energizer catalog record 486, listed as 343, obsolete in the snapshot. Technical data sheet: 343.pdf.
2. [2] Energizer catalog record 552, listed as 387, obsolete in the snapshot. Technical data sheet: 387.pdf.
3. [3] Energizer catalog record 771, listed as E400N, obsolete in the snapshot. No technical data sheet is filed against this record.

Every figure above is reproduced from those records without change; the comparisons are ours and are described on the method page. Snapshot: Energizer technical data, retrieved 1 September 2026.

Putting this on a forum or a repair page? The same answer without the site around it — fit, voltage, chemistry and the limits that go with them — is at /embed/mr42/. It loads nothing from anywhere else. Paste this:

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