A slab serves as a flat surface that is typically horizontal. Used in bridges, floors, buildings roofs, and other types of structures, slabs are supported by walls and reinforced cement beams. These beams are typically cast monolithically along with the slab. It carries the load distribution uniformly and its depth is very small compared to its width and length.
Before we get into the difference between one way slab and two way slab, you should know that there are 5 types of slabs:
 Flat slab
 Oneway slab
 Waffle slabs or grid slabs
 Twoway slab
 Circular slab
What is a Oneway Slab?
Slabs that have only two rigid supports on either side, carry the load on them by flexure in a perpendicular direction to the rigid supports. When a slab is predominantly loaded uniformly, its plane surface distorts into a cylindrical surface. The curvature and the need for the cylindrical surface develop only in one direction. Such slabs are named oneway slabs.
Oneway slabs essentially act as shallow beams having a large width. Slabs that are supported on all 4 sides also behave as a oneway slab if the ratio of their span from longer to shorter is greater than 2 (according to IS 456:2000).
What is a Twoway Slab?
Some slabs receive support on all 4 edges and bear the load by flexure in 2 perpendicular paths, instead of one. They have a less than two longer to shorter span ratio and are called twoway slabs.
Twoway Slab and Oneway Slab: How to Calculate Them
Here are the steps to calculate oneway and twoway slabs:
 If the ratio of their span from longer to shorter is made greater than or equal to 2 then, the slab is regarded as a oneway slab.
L/b (longer span/shorter span) ≥ 2
 On the other hand, if the ratio of their span from longer to shorter is made less than 2, the slab is regarded as a twoway slab.
L/b (longer span/shorter span) < 2
Oneway Slab and Twoway Slab: Difference Between Them
Sl. No.  Point of Comparison  Oneway Slab  Twoway Slab 

Longer span to shorter span ratio (L/b)  Oneway slabs have an L/b ratio that is greater than or equal to two  Twoway slabs have an L/b ratio of less than 2. 

The direction of bending  Slabs bends only in a shorter span direction.  Twoway slabs bend in both directions (longer span and shorter span) 

Reinforcement  The shorter spans receive main reinforcement while the longer spans are provided with distribution reinforcement.  The main reinforcement is delivered in both directions. 

The direction of the crank  The crank is provided in 2 directions.  The crank is provided in all 4 directions. 

Beam support  Beam support is provided to a oneway slab on its two opposite sides.  Twoway slabs are supported by a beam on all 4 sides. 

The direction of the load  Load is carried in only one direction in a shorter span.  Load is carried in both directions. 

Deflected shape  After the bending, the deflected shape of a oneway slab is cylindrical.  After bending, the deformed shape of a twoway slab is a saucer or dishlike shape. 

Examples  Varanda and Chajja are examples of oneway slabs.  Constructive floors of multistory buildings are examples of twoway slabs. 

Amount of steel and thickness  Oneway slabs have less steel in them which is why it has increased depth and thickness.  Twoway slabs have more steel in them which is why it has decreased depth and thickness. 

Economical span  Oneway slabs are economical up to a span of 3.6 metres.  Twoway slabs are economical for panel sizes up to 6m ✕ 6m. 
Oneway and Twoway Slab: Reinforcement Details
Reinforcement Details of Oneway Slab
Suppose, we take a slab measuring 4m in breadth and 10m in length. Then,
L/B = 10/4 = 2.5 ≥ 2
It is clear from the above example that the L/B ratio is greater than 2, making it a oneway slab. The distribution bars are provided in a longer span while the cranked bars or main bars are provided in a shorter span.
Reinforcement Details of Twoway Slab
Let’s take a slab measuring 6m in breadth and 6m in length. Then,
L/B = 6/6 = 1 < 2
It is clear from the example taken that its L/B ratio is less than 2, making it a twoway slab. Here the cranked bars or main bars are provided at a shorter span as well as the longer span too.
The Bottom Line: One way slab and two way slab
The main point of difference between a oneway slab and a twoway slab is that the ratio of their span from longer to shorter will be greater than equal to two for oneway slabs whereas the span ratio of longer to shorter will be less than two (not equal to two) for twoway slabs. Both have their set of differences apart from the L/B ratio (longer span to shorter span). We hope our article on the difference between a oneway slab and a twoway slab helped you spot their differences.