How Accurate Does Your Compass Reading Have to Be for a Vastu Check?

The honest answer is that it depends on where the room sits in your plan, not on how good your phone is. Run the numbers through the grid this site grades against and two figures fall out. A room sitting in the middle of a corner zone keeps its zone through 18.4 degrees of compass error. A room whose centre sits just 2% of the plan's width from a zone line — about 2 mm on a plan printed 10 cm wide — crosses on 3.2 degrees. A careful phone reading, taken outdoors and away from steel, is comfortably inside the first number and nowhere near safe against the second. So the useful question is not "is my compass good enough" but "which of my rooms are close to a line".

Why "the zones are 45 degrees wide" is the wrong way to think about it

Almost every page on Vastu compass accuracy reassures you the same way: the eight directions are 45-degree sectors, so a few degrees cannot matter. That reasoning does not apply to how a floor-plan check works.

The zones are not pie slices radiating from the centre of your flat. They are a nine-cell grid laid over the building's outline — thirds of the width, thirds of the depth — rotated to the north you declare. So what stands between a room and the next zone is a straight line at one-third of the plan's width, and crossing it takes more rotation the further the room already sits from it. The tolerance is a distance, not an angle — which is why two rooms in one flat, on one reading, have wildly different exposure to the same error.

How many degrees each position survives

Computed by running the site's own grid function over a square plan and rotating the declared north by hundredths of a degree until the room changes cell. The inset column is the gap between the room's centre and the nearest zone line, as a fraction of the plan's width, with the millimetre equivalent for a plan printed 10 cm across:

Room centre sits this far inside the zone line	On a 10 cm-wide printout	Compass error it survives
On the line	0 mm	0° — it is already ambiguous
2% of the plan's width	2 mm	3.2°
4%	4 mm	6.1°
6%	6 mm	8.7°
8%	8 mm	11.0°
10%	10 mm	13.0°
15%	15 mm	17.3°
Dead centre of a corner cell (16.7%)	16.7 mm	18.4°

The last figure is not an approximation, it is arithmetic: the centre of a corner cell sits one-third of the plan's width diagonally from the centre of the building, and arctan(1/3) is 18.435 degrees. Any 3×3 grid gives the same number.

Rooms in the middle of an edge cell — north, east, south, west — hold their zone through 45 degrees. The rooms a bad reading puts at risk are always the corner ones, and the corners are where the kitchen, the master bedroom and the toilet usually sit.

One wrong reading moves several rooms at once

This is what makes compass error different from ordinary measurement noise. A wrong bearing does not jostle rooms randomly — it rotates the whole grid, so every room moves the same way at the same time. Take an ordinary 2BHK with its spaces at the middle of their cells — main door north, kitchen south-east, master bedroom south-west, second bedroom north-west, living room north-east, toilet west, balcony east. It scores 94. Hold the rooms still and turn the declared north:

Compass error	What moves	Score
0° to 18.4°	nothing	94
20°	kitchen, master bedroom, second bedroom and living room all cross at once	81
45°	those four plus the toilet	76

Thirteen points disappear in one step, not gradually. The score is not a smooth function of your bearing — it is flat, then it falls off a step. That also means a small error is genuinely harmless and a large one is not merely "a bit less accurate": it is a different plan.

On a real plan, ten degrees often costs nothing

Rooms are not at cell centres in practice, so here is the same exercise on a plan where they sit where rooms actually sit — against the outer walls, with the living room only about 3% of the plan's width inside the north/north-east line.

Between −12° and +12° the score stays at 94. The living room's label does move, from north-east to north, at just +5° — its tolerance is 4.3° — and it costs nothing: the table scores a living room 95 in the north-east and 90 in the north, and both are ideal. The reading was wrong; the verdict was not.

At +15° the kitchen crosses from south-east to east. South-east is 100 for a kitchen, ideal; east is 55, acceptable. The score falls to 84. One room, one line, ten points — because the kitchen is one of the heaviest rooms in the grade.

So a crossing only matters when the cell on the other side is scored very differently. The kitchen's step is a 45-point fall; the living room's is 5 points. Checking the rooms nearest a line against the table tells you whether your reading needs to be good or merely roughly right.

Oblong plans are forgiving in one direction and not the other

On a square plan the tolerance is the same whichever way you are wrong. On a plan one-and-a-half times as long as it is wide, a room at the centre of its corner cell survives 12.5° one way and 26.6° the other; at 2:1, 9.5° and 33.7°; at 3:1, 6.4° and 45°. That is the grid being fitted to a rectangle. On a long, narrow flat an error one way is roughly three times as expensive as the same error the other way, and which way depends on how the plan runs.

Magnetic north, true north, and what to do about the reading

Most traditional Vastu practice in India works in magnetic north; some schools and most architects prefer true north. The gap is the magnetic declination for your location, and across most of India it is small — of the order of a degree or two — though larger towards the far north-east and far west. Against an 18-degree tolerance it will not flip anything. Against a 3-degree one, it can.

The error that bites is interference. A phone's magnetometer is a tiny sensor, pulled off course by steel door frames, reinforcement bars, grilles, meter boxes, magnetic phone cases and the laptop beside you; indoors in a reinforced concrete building you are standing inside a steel cage. Take the bearing outside the building, away from metal, then transfer it to the plan — our free online Vastu compass has the declination field and how to take a reading that does not drift, and finding north on a house plan covers the case where the drawing's own arrow disagrees with your phone.

What this does not settle

Getting the bearing right fixes the input, not the resolution of the model. Our grid resolves to nine cells, while the classical 32-pada systems divide each wall into eight positions of 11.25 degrees — finer than the error most instruments make, which is one reason we grade zones and not padas (where on the wall the main door sits). And no bearing tells a check anything about the slope of the site, the road level, what sits above or below a room, or the plot's surroundings. Those need eyes on the place.

Check your own plan

Upload your floor plan for a free Vastu check: rooms are detected on the plan, you confirm which way is north and the room labels, and the free check shows the direction-wise zones and a score. Set north as carefully as you can, then look at which rooms sit nearest a zone line — those are the only ones your reading can change. The detailed report adds room-by-room analysis and remedies ranked by what they can actually change.