A crowded row can contain many more seedlings than the final planting pattern allows. Before removing any, work out where the retained plant centres will sit. The distance along the row and the clearance at both ends matter just as much as the chosen spacing.
Use Seedling Thinning Calculator to compare a straight row's available positions with its current seedling count. It reports the maximum spaced positions, seedlings above that capacity and any shortfall when the current count is lower.
A four metre row at twenty centimetres
For a 4 m row, leave 10 cm clear at each end. The usable centre span is 3.8 m. At 20 cm between centres, there are 19 intervals and 20 plant positions because both endpoints are included when the spacing fits exactly.
If 45 seedlings are currently in the row, the calculator reports 25 above that capacity. If there are no seedlings, the same geometry reports 20 unfilled positions. These counts describe the row capacity; they do not identify which individual seedlings should be retained.
The formula counts centres, not intervals
Dividing 3.8 m by 0.2 m gives 19. That is the number of equal gaps, not the number of plants placed across those gaps. Adding the endpoint plant produces the capacity of 20.
This distinction is easy to miss in short rows. The tool floors a nonexact interval count, then adds the first possible centre. If the clear ends leave no usable span, the result reflects that geometry rather than trying to fit plants by area.
The setback field is in centimetres while total row length is in metres. Converting the row length before entering it again can introduce a factor of 100 error. Follow the units printed beside each field instead.
Capacity does not choose the seedlings
A count alone cannot assess root disturbance, seedling condition or crop suitability. It also assumes that the retained centres can be arranged at the specified spacing along a straight row. Real seedlings may be clustered in some stretches and missing in others, so inspect their actual positions before treating the excess count as a removal plan.
Keep the final spacing tied to the crop and your chosen growing method. The example of 20 cm demonstrates counting, not a universal spacing for vegetables.
For a full rectangular planting grid with several rows, Plant Spacing Calculator is a better fit. It compares row directions and edge setbacks across a bed. If the empty positions will be filled from a new seed batch, Seed Quantity Calculator can estimate the sowing requirement without pretending every seed becomes a retained plant.